BIOLOGY 
LIBRARY 

G 


. 


PREFACE 

The  present  volm  brings  together  in  brief  form 
fundamental  priix  ipl»  >f  biology  for  the  college  student 
the  general  reader 

It  is  well  recogni/i  that  there  is  no  adequate  subst 
for  detailed  laborator  vork  on  the  structure  and  physic 
of  representative  ory  -m>  as  a  means  of  affording  a 
hand  knowledge  of  ;|  acts  and  methods  of  biology.  I 
ever,  the  author  I  ili/cd  with  increasing  force  thai 
student's  correlation  the  laboratory  data  from  day  to 
and  accordingly  hi-  preciation  of  the  broader  aspec 
the  subject  are  great  enhanced  by  a  synchronous  'run 
account'  of  the  um  lying  principles.  The  material 
this  volume  has  pro1  to  be  of  great  value  for  this  pur 
in  a  course*  on  (iencr  Biology  elected  each  year  by  se\ 
hundred  Yale  underg  dilates. 

The  large  problems  f  life  are  common  to  both  zoology 
botany,  and  therefoi  both  animals  and  plants  have  1 
drawn  upon  for  illust  tion  and  discussion.  This  inetho 
presentation  accords  ith  the  author's  conviction  that 
general  biological  vie  point  is  the  most  favorable  mean 
approach  both  to  a  road  knowledge  of  living  phenon 
as  a  part  of  a  'lilH'il1  education,  and  to  more  advai 
studies  in  zoology  ad  botany  which  are  prerequisite 


FOUNDATIONS  OF  BIOLOGY 


BY 
LORANDE  LOSS  ^OODRUFF 

PROFESSOR   OF    BIOLOGY   IN   YALE   UNIVERSITY 


J^eto  gorfe 

THE  MACMILLAN  COMPANY 
1922 

All  rights  reserved 


FOUNDATIONS  OF  BIOLOGY 


BY 
LORANDE  LOSS  ^OODRUFF 

PROFESSOR   OF    BIOLOGY   IN   YALE   UNIVERSITY 


THE  MACMILLAN  COMPANY 
1922 

All  rights  reserved 


FOUNDATIONS  OF  BIOLOGY 


BY 

LORANDE  LOSS 

PROFESSOR   OF   BIOLOGY   IN   YALE   UNIVERSITY 


J^teto 

THE  MACMILLAN  COMPANY 
1922 

All  rights  reserved 


PRINTED   IN   THE    UNITED   STATES   OF   AMERICA 


BIOLOGY 

LIBRARY 

6 


COPYRIGHT,  1922, 
BY  THE  MACMILLAN  COMPANY 

S«t  up  and  electrotyped.     Published  June,  1922 


PREFACE 

The  present  volume  brings  together  in  brief  form  the 
fundamental  principles  of  biology  for  the  college  student  and 
the  general  reader. 

It  is  well  recognized  that  there  is  no  adequate  substitute 
for  detailed  laboratory  work  on  the  structure  and  physiology 
of  representative  organisms  as  a  means  of  affording  a  first- 
hand knowledge  of  the  facts  and  methods  of  biology.  How- 
ever, the  author  has  realized  with  increasing  force  that  the 
student's  correlation  of  the  laboratory  data  from  day  to  day 
and  accordingly  his  appreciation  of  the  broader  aspects  of 
the  subject  are  greatly  enhanced  by  a  synchronous  ' running 
account'  of  the  underlying  principles.  The  material  in 
this  volume  has  proved  to  be  of  great  value  for  this  purpose 
in  a  course  on  General  Biology  elected  each  year  by  several 
hundred  Yale  undergraduates. 

The  large  problems  of  life  are  common  to  both  zoology  and 
botany,  and  therefore  both  animals  and  plants  have  been 
drawn  upon  for  illustration  and  discussion.  This  method  of 
presentation  accords  with  the  author's  conviction  that  the 
general  biological  viewpoint  is  the  most  favorable  means  of 
approach  both  to  a  broad  knowledge  of  living  phenomena 
as  a  part  of  a  'liberal'  education,  and  to  more  advanced 
studies  in  zoology  and  botany  which  are  prerequisite  for 


rr  o  f  i»  rr  •» 


VI  PREFACE 

medicine,  forestry,  etc.  As  is  natural,  however,  the  zoolog- 
ical aspect  has  been  emphasized  since  it  affords  indispensable 
data  for  the  interpretation  of  Man  himself.  For  courses  in 
general  zoology,  therefore,  the  book  will  be  found  adequate 
in  its  treatment  of  animals,  while  chapters  VIII  and  IX  on 
plants  may  readily  be  omitted,  without  breaking  the  con- 
tinuity of  the  discussion. 

The  author  is  indebted,  of  course,  to  innumerable  sources 
for  the  facts  and  principles  outlined.  The  content  has 
grown  by  accessions  year  by  year.  Nearly  all  the  stand- 
ard treatises  have  been  drawn  upon,  but  those  which 
have  been  most  generally  suggestive  are  listed  in  the  bibli- 
ographies of  the  respective  chapters.  Specific  mention,  how- 
ever, should  be  made  here  of  Professor  Wilder's  History  of 
the  Human  Body,  Professor  Conklin's  Heredity  and  En- 
vironment in  the  Development  of  Men,  Professor  Ganong's 
Text-book  of  Botany,  and  Professor  Coulter's  Evolution  of 
Sex  in  Plants. 

The  author  has  availed  himself  of  the  constructive  criticism 
generously  given  by  Professor  B.  W.  Kunkel  of  Lafayette 
College,  Professor  E.  H.  Cameron  of  the  University  of 
Illinois,  and  his  colleagues  at  Yale,  Professors  R.  G.  Harrison, 
W.  R.  Coe,  A.  Petrunkevitch,  F.  P.  Underbill,  Henry 
Laurens,  G.  A.  Baitsell,  W.  W.  Swingle,  and  Dr.  J.  W. 
Buchanan,  who  have  read  the  book  either  in  manuscript  or 
in  the  mimeographed  form  in  which  it  has  been  used  by  the 
Yale  classes.  And  Professor  Baitsell's  interest  in  the  work 
of  the  course  has  made  it  possible  to  impose  upon  him  the 
added  task  of  reading  the  book  at  each  stage  of  its  develop- 
ment. Miss  Hope  Spencer  of  the  Yale  Laboratory  has  as- 
sumed with  enthusiasm  a  considerable  portion  of  the  editorial 
work  involved  in  seeing  the  book  through  the  press. 
Finally,  the  author's  indebtedness  to  the  criticism  and 


PREFACE  Vll 

cooperation  of  his  wife,  Margaret  Mitchell  Woodruff,  must 
not  remain  unmentioned,  though  it  cannot  be  adequately 
expressed. 

The  original  illustrations  as  well  as  those  from  other  sources 
which  have  been  modified  or  merely  redrawn  are,  with  a 
few  exceptions,  the  work  of  Mr.  R.  E.  Harrison,  Yale,  1923. 
In  most  cases  these  figures  have  been  selected  because  of 
their  proved  pedagogic  value.  Acknowledgments  are  due  to 
the  authors  and  publishers  of  the  following  works,  from  which 
illustrations  have  been  reproduced  by  permission:  Coulter, 
Barnes,  and  Cowles'  Textbook  of  Botany,  Coulter's  Plant 
Life  and  Plant  Uses  (American  Book  Co.) ;  Kellicott's  Social 
Direction  of  Human  Evolution,  Jordan  and  Kellogg's  Evolu- 
tion and  Animal  Life,  Darwin's  Life  and  Selected  Letters, 
Huxley's  Life  and  Letters  (D.  Appleton  &  Co.);  Folsom's 
Entomology,  Gager's  Fundamentals  of  Botany  (P.  Blakis- 
ton's  Sons  &  Co.);  Jennings'  Behavior  of  the  Lower  Organ- 
isms (Columbia  University  Press);  Bergen's  Foundations 
of  Botany,  Bergen  and  Caldwell's  Practical  Botany,  Bergen 
and  Davis'  Principles  of  Botany,  Densmore's  General 
Botany,  Hough  and  Sedgwick's  The  Human  Mechanism, 
Linville  and  Kelly's  General  Zoology  (Ginn  &  Co.);  Kelli- 
cott's  General  Embryology,  Sedgwick  and  Wilson's  General 
Biology  (Henry  Holt  &  Co.);  Morgan's  Physical  Basis  of 
Heredity  (J.  P.  Lippincott  &  Co.);  Romanes'  Darwin  and 
After  Darwin  (Open  Court  Publishing  Co.);  Conklin's 
Heredity  and  Environment  in  the  Development  of  Men 
(Princeton  University  Press) ;  Conn  and  Budington's  Physi- 
ology and  Hygiene  (Silver,  Burdett  &  Co.) ;  Coulter's  Evolu- 
tion of  Sex  in  Plants  (University  of  Chicago  Press) ;  Abbott's 
General  Biology,  Buchanan  and  Buchanan's  Bacteriology, 
Campbell's  University  Textbook  of  Botany,  Ganong's  Text- 
book of  Botanv  for  Colleges,  Hegner's  College  Zoology,  and 


Vlll  PREFACE 

Introduction  to  Zoology,  Holmes'  Biology  of  the  Frog,  Hux- 
ley's Physiology,  Lankester's  Treatise  on  Zoology,  Lull's 
Organic  Evolution,  Packard's  Textbook  of  Entomology, 
Parker's  The  Elementary  Nervous  System,  Parker  and  Has- 
well's  Textbook  of  Zoology,  Parker  and  Parker's  Practical 
Zoology,  Scott's  The  Theory  of  Evolution,  Shipley  and 
McBride's  Zoology,  Verworn's  General  Physiology,  Walter's 
Genetics,  and  The  Human  Skeleton,  Wiedersheim's  Compar- 
ative Anatomy  of  Vertebrates,  Wilson's  The  Cell  (The  Mac- 
millan  Co.).  To  The  Macmillan  Company  is  also  due  the 
author's  appreciation  of  the  liberal  attitude  which  they  have 
assumed  in  all  the  arrangements  attendant  upon  the  pub- 
lication of  the  book. 

L.  L.  WOODRUFF. 

Yale  University,  , 

May,  1922. 


CONTENTS 

CHAPTER  PAGE 

I.     THE  SCOPE  OF  BIOLOGY 1 

II.     THE  PHYSICAL  BASIS  OF  LIFE 6 

A.  Protoplasm 7 

B.  Characteristics  of  Living  Matter 10 

1.  Chemical  Composition 11 

2.  Metabolism 15 

3.  Growth 16 

4.  Reproduction 17 

5.  Adaptation 17 

6.  Organization 18 

III.     ORGANIZATIONAL  UNITS  OF  PLANTS  AND  ANIMALS       .  21 

A.  The  Cell 23 

1.  Cytoplasm 24 

2.  Nucleus 27 

B.  Origin  of  Cells 28 

TV.     METABOLISM  OF  GREEN  PLANTS 30 

A.  Structure  and  Life  History  of  Sphaerella   ...  30 

B.  Metabolism  in  Sphaerella 34 

1.  Food  Making 35 

2.  Respiration 37 

V.     METABOLISM  OF  ANIMALS 39 

A.  Structure  and  Life  History  of  Paramecium      .      .  39 

B.  Metabolism  in  Paramecium 41 

1.  Food  Taking 42 

2.  Respiration  and  Excretion 43 

VI.     METABOLISM  OF  COLORLESS  PLANTS 44 

A.  The  Bacteria 44 

B.  Cycle  of  the  Elements  in  Nature 46 

C.  The  Hay  Infusion  Microcosm 50 

VII.     THE  MULTICELLULAR  ORGANISM 54 

VIII.     THE  PLANT  BODY 61 

A.   Gross  Structure 65 

1.  Root 65 

2.  Stem 69 

3.  Leaf  .  71 


X  CONTENTS 

CHAPTER  PAGE 

VIII.     THE  PLANT  BODY — Continued 

B.  Histology 75 

1.  Root 78 

2.  Stem 81 

3.  Leaf 82 

C.  Physiology 84 

1.  Circulation  Paths 85 

2.  Dynamics  of  Circulation 88 

3.  Food  Utilization 89 

IX.     REPRODUCTION  IN  PLANTS 91 

A.  Spore  Formation 92 

B.  Gamete  Formation 94 

C.  Sex  Differentiation 96 

D.  Reproductive  Organs 98 

E.  Alternation  of  Generations 100 

1.  The  Moss 100 

2.  The  Fern 103 

3.  Higher  Ferns 105 

4.  Flowering  Plants 107 

X.     THE  ANIMAL  BODY 115 

A.  The  Chief  Groups  of  Animals     . 116 

B.  Hydra 118 

C.  Earthworm 121 

D.  Crayfish 129 

E.  Vertebrates 135 

1.  Body  Plan 136 

2.  Skin 138 

3.  Muscles 138 

4.  Coelom 140 

5.  Skeleton 140 

F.  Diagnostic  Vertebrate  Characters 146 

XI.     NUTRITION  IN  ANIMALS 154 

A.  The  Alimentary  Canal     . 154 

B.  Digestion 157 

XII.     CIRCULATION  AND  RESPIRATION  IN  ANIMALS     .      .      .  161 

A.  Circulation  in  the  Lower  Vertebrates    .      .      .      .  162 

B.  Respiration 168 

C.  Circulation  in  the  Higher  Vertebrates  .      .      .      .  170 

XIII.  EXCRETION  IN  ANIMALS 175 

XIV.  COORDINATION  IN  ANIMALS 181 

A.  Chemical  Coordination 181 

B.  Coordination  by  the  Nervous  System  ....  183 


CONTENTS  xi 

CHAPTER  PAGE 

XIV.  COORDINATION  IN  ANIMALS — Continued 

C.   Sense  Organs 193 

1.  Cutaneous  Senses 195 

2.  Sense  of  Taste 195 

3.  Sense  of  Smell 196 

4.  The  Ear 196 

5.  The  Eye 198 

XV.  REPRODUCTION  IN  ANIMALS 203 

XVI.     ORIGIN  OF  THE  INDIVIDUAL 209 

A.  Origin  of  Life 209 

B.  Reproduction 212 

C.  Origin  of  the  Germ  Cells 223 

1.  Mitosis 224 

2.  Gametes 228 

3.  Spermatogenesis 230 

4.  Oogenesis 232 

5.  The  Chromosome  Cycle 233 

6.  Fertilization 237 

D.  Significance  of  Fertilization 242 

1.  Protista 243 

2.  Metazoa 249 

E.  Organization  of  the  Zygote 251 

XVII.     HERITAGE  OF  THE  INDIVIDUAL 261 

A.  Heritability  of  Variations 264 

1.  Modifications 265 

2.  Combinations 268 

3.  Mutations 269 

B.  Galton's  'Laws' 269 

C.  Mendelisrn 271 

1.  Monohybrids 272 

2.  Dihybrids 276 

3.  Trihybrids 280 

4.  General  Principles 280 

D.  Neo-Mendelism 282 

E.  Mechanism  of  Mendelian  Inheritance  ....  287 

1.  Sex  Determination 291 

2.  Linkage 293 

F.  Nature  versus  Nurture    .      .  '.-•-• 296 

G.  Selection .      .      .    -.      .  299 

Pure  Lines .      .  393 

Summary 306 


Xll 


CONTENTS 


CHAPTER 

XVIII. 


XIX. 


ADAPTATION  OF  ORGANISMS 

A.  Adaptations  to  the  Physical  Environment 

1.  Adaptations  Essentially  Functional  . 

Food 

Temperature 

Pressure 

2.  Adaptations  Essentially  Structural    . 

Adaptive  Radiation  of  Mammals  . 

Animal  Coloration 

The  Legs  of  the  Honey  Bee 

B.  Adaptations  to  the  Living  Environment    . 

1.  Communal  Associations 

2.  Symbiosis 

3.  Parasitism 

4.  Immunity 

C.  Individual  Adaptability 

THE  ORIGIN  OF  SPECIES 

A.  Evidences  of  Organic  Evolution 

1.  Taxonomy 

2.  Comparative  Anatomy 

3.  Paleontology 

4.  Embryology 

5.  Physiology 

6.  Distribution 

B.  Factors  of  Organic  Evolution 

EPOCHS  IN  BIOLOGICAL  HISTORY 

A.  Greek  and  Roman  Science 

B.  Medieval  and  Renaissance  Science 

C.  The  Microscopists 

D.  The  Development  of  the  Subdivisions  of  Biology 

1.  Taxonomy 

2.  Comparative  Anatomy 

3.  Physiology 

4.  Histology 

5.  Embryology 

6.  Genetics 

7.  Organic  Evolution 


APPENDIX 


I.    CLASSIFICATION 

A.  Plants  . 

B.  Animals 
II.  BIBLIOGRAPHY 

III.  GLOSSARY 


PAGE 

307 

308 

308 

308 

311 

313 

313 

313 

319 

324 

330 

331 

331 

334 

338 

339 

345 

347 

348 

351 

356 

364 

367 

368 

372 

379 

379 

382 

386 

389 

390 

392 

394 

398 

401 

403 

406 


413 
413 
414 
417 
429 


INDEX 


457 


LIST   OF   ILLUSTRATIONS 

FIGURE                                                                                    TITLE  PAGE 

Charles  Darwin Frontispiece 

1  The  divisions  of  biology 4 

2  Amoeba  proteus 9 

3  Alveolar  appearance  of  protoplasm 10 

4  Cells 21 

5  Section  of  a  leaf 22 

6  Section  of  Hydra 22 

7  Types  of  cells 25 

8  Diagram  of  a  cell 26 

9  Life  cycle  of  Sphaerella 32 

10  Paramecium  calkinsi 40 

11  Paramecium  aurelia,  dividing 41 

12  Paramecia  conjugating 42 

13  Types  of  Bacteria 45 

14  Types  of  flagellation  in  Bacteria 46 

15  The  carbon  cycle 48 

16  The  nitrogen  cycle 49 

17  Spondylomorum 55 

18  Volvox  globator 56 

19  Cleavage  of  Sea  Urchin's  egg 58 

20  Section  of  Frog's  intestine 59 

21  Cross  section  of  a  plant  stem 60 

22  Spirogyra 61 

23  Common  Seaweed  (Fucus) 62 

24  Giant  Kelp r 63 

25  Gulf  weed  (Sargassum) 64 

26  Types  of  roots O     ...  65 

27  Seasonal  history  of  an  annual  plant 66 

28  Seasonal  history  of  a  biennial  plant 67 

29  Aerial  roots  of  English  Ivy 67 

30  Haustoria  of  Dodder 68 

31  Strawberry  runners 69 

32  Hyacinth  bulb 70 

33  'Smilax'  (Myrsiphyllum) 71 

34  Leaf  of  a  Flowering  Plant                      71 


XIV  LIST   OF   ILLUSTRATIONS 

FIGURE                                                                                    TITLE  PAGE 

35  Winter  buds 72 

36  Onion  leaf 72 

37  Pitcher  Plant  (Sarracenia  purpurea) 73 

38  Sundew 73 

39  Sensitive  Fern  (Onoclea  sensibilis) 74 

40  Floral  parts  of  Azalea  (Loiseleuria) 75 

41  Ideal  vertical  section  of  a  Flowering  Plant 76 

42  Generalized  plant  cell 77 

43  Root  tip 79 

44  Root  hair 80 

45  Cross  section  of  a  stem 81 

46  A  bud  of  Elodea  canadensis 82 

47  Cross  section  of  a  leaf 83 

48  Diagram  illustrating  the  physiology  of  a  plant     ....  87 

49  Ulothrix 95 

50  Oedogonium,  filamentous  form 97 

51  Oedogonium,  zygote 98 

52  A  Brown  Alga  (Ectocarpus) 99 

53  Life  history  of  a  Moss 101 

54  Life  history  of  a  Fern 104 

55  Decline  of  gametophyte  and  increase  of  sporophyte           .     .  105 

56  Life  history  of  a  higher  Fern 106 

57  Megaspore  and  microspore 107 

58  Typical  flower 107 

59  Transition  between  petals  and  stamens 108 

60  Union  of  carpels  to  form  the  ovule  case 108 

61  The  life  history  of  a  higher  Flowering  Plant 110 

62  Seed  of  Violet Ill 

63  Diagram  of  comparative  morphology  of  plants    .      .      .      .  112 

64  Hydra,  longitudinal  section 119 

65  Hydra,  transverse  section 120 

66  Earthworm,  body  plan 122 

67  Dissection  of  Earthworm  (Lumbricus  terrestris)     ....  123 

68  Earthworm,  transverse  section  . 124 

69  Stages  in  development  of  the  Earthworm 126 

70  Structure  of  a  primitive  Arthropod 130 

71  Dissection  of  Crayfish  (Cambarus  affinis) 131 

72  Appendages  of  Crayfish 132 

73  Nervous  system  of  Earthworm  and  Crayfish        ....  134 

74  Sagittal  section  of  an  ideal  Vertebrate 137 

75  Cross  section  of  an  ideal  Vertebrate 137 

76  Section  of  human  skin 139 

77  Skeleton  of  a  bony  Fish  (Perca  fluviatilis) 141 


LIST   OF   ILLUSTRATIONS  XV 

FIGURE                                                                                    TITLE  PAGE 

78  Relation  of  the  notochord  to  vertebrae 142 

79  A  typical  human  vertebra 143 

80  Plan  of  th3  Vertebrate  limb  skeleton 144 

81  Skeleton  of  a  Mammal  (Felis  domesticd) 145 

82  Dissection  of  Yellow  Perch  (Perca  flavescens)        ....  148 

83  Dissection  of  Green  Frog  (Rana  clamitans) 149 

84  Dissection  of  Pine  Lizard. (Sceloporus  undulatus)       .      .      .  150 

85  Dissection  of  domestic  Pigeon  (Columba  livid]      ....  151 

86  Dissection  of  Gray  Squirrel  (Sciurus  carolinensis)      .      .      .  152 

87  Sagittal  section  of  human  body 153 

88  Human  alimentary  canal  and  derivatives 155 

89  Chemical  activities  of  the  digestive  tract 158 

90  A  gland 159 

91  Vascular  system  of  a  Shark        .      .      . 164 

92  Plan  of  the  circulatory  system  of  a  Fish,  Amphibian,  and 

Mammal 165 

93  Paths  of  absorbed  food 168 

94  Food  and  respiratory  paths        .      . 170 

95  Transformations  of  the  aortic  arches 171 

96  Nephridium  of  an  Earthworm 177 

97  Evolution  of  the  urogenital  system 179 

98  Section  of  human  kidney  and  ureter 180 

99  Simple  receptor-effector  system 184 

100  More  complex  receptor-effector  system 184 

101  Simple  reflex  arc 184 

102  Differentiation  of  nerve  cells 185 

103  Nervous  organization  of  intestinal  wall 186 

104  Development  of  the  Vertebrate  brain 187 

105  Types  of  Vertebrate  brains 189 

106  Nervous  system  of  the  Frog 190 

107  Paths  of  nervous  impulses 193 

108  Differentiation  of  sense  cells 194 

109  Membranous  labyrinth  of  the  ear 197 

110  Human  ear 198 

111  Development  of  the  eye 200 

112  Vertebrate  eye  (human) 201 

113  Uterus  (human) 205 

114  Yeast  cells 213 

115  Reproductive  cell  cycles 214 

116  Hydra,  dividing 217 

117  Flatworm,  dividing 217 

118  Obelia 218 

119  Body  plan  of  polyp  and  medusa 219 


XVI  LIST   OF   ILLUSTRATIONS 

FIGUBE                                                                                    TITLE  PAGE 

120  Regeneration  and  grafting  in  Hydra 221 

121  Regeneration  and  grafting  in  an  Earthworm 222 

122  Regeneration  and  grafting  in  a  Flatworm 223 

123  Typical  stages  in  Mitosis 225 

124  Period  of  chromosome  reduction  in  animals  and  plants       .  229 

125  Spermatogenesis  and  oogenesis  in  animals 231 

126  Chromosome  cycle  in  an  animal 235 

127  Egg  and  sperm  of  Lamprey 236 

128  Hen's  egg 238 

129  Human  egg  and  sperm 239 

130  Conjugation  in  Paramecium 245 

131  Endomixis  in  Paramecium •     .  248 

132  Development  of  Dentalium 255 

133  Comparison  of  the  development  of  Dentalium  and  Amphioxus  257 

134  Relation  of  cytoplasmic  differentiation  to  development        .  259 

135  Continuity  of  the  germ  plasm 265 

136  Alternative,  mosaic,  and  blending  inheritance      ....  269 

137  Law  of  filial  regression 270 

138  Inheritance  of  size  in  Peas 273 

139  Mendelian  monohybrid 275 

140  Mendelian  dihybrid 277 

141  Inheritance  of  human  hair  characters 278 

142  Possible  types  of  zygotes  in  a  dihybrid 279 

143  Mendelian  trihybrid        281 

144  Color  inheritance  in  the  Four-o'clock  (Mirabilis  jalapa)       .  284 

145  Color  inheritance  in  mulattoes 285 

146  Chromosome  cycle  in  an  animal 289 

147  Chromosomes  or  genes  at  fertilization  and  maturation  .      .  290 

148  The  X  chromosome  in  fertilization 293 

149  Inheritance  of  color-blindness  from  the  male 294 

150  Inheritance  of  color-blindness  from  the  female     ....  295 

151  Crossing-over  at  synapsis 296 

152  Nature  and  nurture 298 

153  Population  and  pure  lines  in  Beans 300 

154  Model  to  illustrate  the  law  of  probability 301 

155  Normal  frequency  curve 302 

156  Selection  and  filial  regression 303 

157  Relation  of  pure  lines  to  a  population 305 

158  Sulfur  Bacteria  (Beggiotoa) 309 

159  Yeast 310 

160  Spore  formation  in  Bacteria             312 

161  Adaptive  radiation  in  Eutheriau  Mammals 314 

162  Gymnura ....  315 


LIST    OF    ILLUSTRATIONS  XVU 

FIGURE                                                                                   TITLE  PAGE 

163  Foot  postures  of  Mammals 316 

164  Sloth  (Choloepus  hoffmanni)    .          317 

165  Mole  (Talpa  europea) 317 

166  Porpoise  (Phocaena  communis) 317 

167  'Flying  Lemur'  (Galeopithecus  volans) 318 

168  Bat  (Vespertilio  noctula) 318 

169  Katydid  (Microcentrum  laurifolium) 319 

170  Underwing  Moth  (Catocala  lacrymosa) 320 

171  Dead-leaf  butterfly  (Kallima  paralecta) 321 

172  Walking-stick  (Diapheromera  femorata) 322 

173  Larva  of  a  Geometrid  Moth 322 

174  Drone  Bee  and  Bee  Fly  (Eristalis  tenax) 323 

175  Bees 325 

176  Head  of  Bee 326 

177  Legs  of  Bee 327 

178  Foot  of  Bee 328 

179  Formation  of  a  Lichen 332 

180  Ants  and  Aphids 333 

181  Life  cycle  of  a  Malarial  Parasite 335 

182  Trypanosoma  theileri 337 

183  Avoiding  reaction  of  Paramecium 340 

184  Rotation  of  Paramecium 341 

185  Vertebrate  fore-limbs 352 

186  Skeleton  of  Man  and  Gorilla 354 

187  Vestigial  hind-limbs  of  a  Snake 355 

188  Archaeopteryx 360 

189  Evolution  of  the  Horse 362 

190  Evolution  of  the  Camel 363 

191  Comparison  of  embryos  of  Fish,  Bird,  and  Man        .      .      .  366 

192  Phylogeny  of  the  Elephants 370 

193  Evolution  of  the  head  of  Elephants 371 

194  Varieties  of  domestic  Pigeons 373 

195  Aristotle 380 

196  Theophrastus  of  Eresus 381 

197  Andreas  Vesalius 384 

198  William  Harvey 386 

199  Antony  van  Leeuwenhoek 387 

200  Marcello  Malpighi 389 

201  Carolus  Linnaeus 391 

202  Georges  Cuvier 392 

203  Thomas  Henry  Huxley 393 

204  Stephen  Hales 397 

205  Matthias  Jacob  Schleiden  399 


XVili  LIST    OF   ILLUSTRATIONS 

FIGUBE                                                                                    TITLE  PAGE 

206  Theodor  Schwann 400 

207  Karl  Ernst  von  Baer 402 

208  Gregor  Johann  Mendel 405 

209  Comte  de  Buffon 407 

210  Erasmus  Darwin 408 

211  Jean-Baptiste  Lamarck 409 


FOUNDATIONS  OF   BIOLOGY 


FOUNDATIONS  OF  BIOLO.GY 

CHAPTER  I 
THE  SCOPE  OF  BIOLOGY 

Science  is,  in  its  source,  eternal;  in  its  scope,  unmeasurable; 
in  its  problem,  endless;  in  its  goal,  unattainable.  — von  Baer. 

THE  oldest  and  the  most  obvious  classification  of  the 
materials  of  our  environment  is  into  non-living  and  living; 
and  the  accumulation  of  knowledge  in  regard  to  the  former  is 
represented  in  the  so-called  physical  sciences,  while  that  of 
the  latter  comprises  the  content  of  BIOLOGY,  the  science  of 
matter  in  the  livine^state..  Biology,  like  all  science,  has  as  its 
ultimate  object  the  explanation  of  its  phenomena  in  terms 
of  the  basic  concepts — matter  and  energy  acting  in  space 
and  time;  but  it  is  needless  to  say  that  the  realization  of  this 
object  is  not  imminent  in  any  department  of  knowledge,  and 
least  of  all  in  the  science  of  living  things  which  exhibit  a 
condition  of  matter  which  altogether  transcends  the  classi- 
fications of  physicist  and  chemist  to-day  —  a  condition  which 
expresses  in  its  highest  manifestations  what  we  call  'our  life.' 

Whether  the  'riddle  of  life'  will  ultimately  be  solved  is  a 
question  which  every  one  would  like  to  answer  but  only  the 
rash  would  attempt  to  predict.  Suffice  it  to  say  that  biolo- 
gists who  are  on  the  firing  line  of  progress  to-day  are  directing 
their  attention  solely  to  an  attempt  to  elucidate  life  phenom- 
ena in  terms  which  the  chemist  and  physicist  offer.  Our 
present  interest,  however,  is  not  in  discussing  the  theoretical 
goal  of  biology,  but  in  drawing  in  bold  strokes  an  outline 
picture  of  the  present-day  knowledge  of  the  subject  which 

1 


2  FOUNDATIONS   OF   BIOLOGY 

represents  the  cumulative  results  of  the  application,  to 
problems  of  life,  of  the  Scientific  method  —  a  method  which 
is  not  peculiar  to  science  but  merely  a  perfected  concentra- 
tion of  our  human  resources  of  observation,  experimentation, 
and  reflection.  Thus  far  this  has  been  a  most  productive 
method  and  certainly  has  given  no  evidence  that  its  useful- 
ness is  being  exhausted.  To  follow  any  other  course  would 
be  to  abandon  the  method  of  science.  "In  ultimate  analysis 
everything  is  incomprehensible,  and  the  whole  object  of 
science  is  simply  to  reduce  the  fundamental  incomprehensi- 
bilities to  the  smallest  possible  number." 

The  foundations  of  the  scientific  study  of  living  nature  were 
laid  by  Aristotle  and  Theophrastus  over  2000  years  ago.  On 
the  basis  of  collecting,  dissecting,  classifying,  and  pondering 
they  reached  generalizations,  many  of  which  have  but  recently 
been  put  on  a  firm  basis  of  fact.  Indeed  these  pioneers 
asked  nearly  all  the  broad  questions  which  are  fundamental 
to-day;  but  from  the  Greeks  until  about  the  fifteenth  century 
there  is  little  to  record.  There  were  many  additions  to  the 
body  of  knowledge  during  this  long  slumber  period,  but  fact 
and  fancy  were  so  amalgamated  that  the  truth  was  obscured. 

The  feeling  that  though  Man  is  of  nature,  he  is  still  apart, 
was  expressed  at  the  revival  of  learning  in  the  broad  classifica- 
tion of  all  knowledge  as  history  of  nature  and  history  of 
Man;  the  former  having  as  its  content  the  record  or  "history 
of  such  facts  or  effects  of  nature  as  have  no  dependence  on 
Man's  will,  such  as  the  histories  of  metals,  plants,  animals, 
regions,  and  the  like";  the  latter  treating  of  the  voluntary 
actions  of  men  in  communities.  Thus  all  record  of  facts 
was  either  natural  history  or  civil  history.  From  this  more 
or  less  nebulous  natural  history  the  present-day  sciences  of 
astronomy,  physics,  chemistry,  geology,  and  biology  were 
thrown  off  as  relatively  independent  bodies  of  facts  as  each 


THE    SCOPE    OF   BIOLOGY  3 

gained  content,  clearness,  and  individuality.  Astronomy, 
physics  or  natural  philosophy,  and  chemistry  were  emanci- 
pated first  owing  to  the  fact  that  their  material  was  more 
readily  susceptible  to  mathematical  and  experimental  treat- 
ment, thus  leaving  the  histories  of  the  Earth,  animals  and 
plants,  or  so-called  observational  sciences,  as  the  residue  for 
natural  history.  It  is  in  this  restricted  sense  that  natural 
history  still  lingers. 

It  remained,  however,  for  Lamarck  and  Treviranus  during 
the  opening  years  of  the  nineteenth  century  to  attain  a  vision 
of  the  unity  of  animal  and  plant  life  —  the  unity  of  ZOOLOGY 
and  BOTANY  —  and  to  express  it  in  the  term  biology.  But 
biology  is  something  more  than  a  union  of  botany  and 
zoology  under  one  name  —  for  it  endeavors,  in  addition  to 
describing  the  characteristics  of  animals  and  plants,  to  un- 
fold the  general  principles  underlying  both. 

Thus  the  biologist  has  as  his  field  the  study  of  living 
things  —  what  they  are,  what  they  do,  and  how  they  do 
it.  He  asks,  how  this  animal  or  that  plant  is  constructed 
and  how  it  works  —  and  this  he  attempts  to  answer. 
He  would  like  to  ask,  why  it  is  so  constructed  and  why 
it  works  the  way  it  does  —  but  this  is  beyond  the  scope 
of  science. 

These  queries  of  the  biologist  reflect  the  two  primary 
viewpoints  from  which  biological  phenomena  may  be  ap- 
proached: the  morphological  in  which  interest  centers; 
upon  the  form  and  structure  of  living  things,  and  the; 
physiological  in  which  attention  is  concentrated  upon  the 
functions  performed  —  the  mechanical  and  chemical  engin- 
eering of  living  machines.  Clearly,  however,  it  is  impossible 
to  draw  a  hard  and  fast  distinction  between  morphology  and 
physiology  because  in  the  final  analysis  structure  must  be 
interpreted  in  terms  of  function,  and  vice  versa.  But  again, 


4  FOUNDATIONS    OF   BIOLOGY 

the  fields  of  morphology  and  physiology  naturally  resolve 
themselves  into  special  departments  of  study,  depending  on 
the  level  of  analysis  of  structure  or  of  function  which  is  em- 
phasized. Thus  MOBPHOLOGY  stresses  the  general  form  of 
the  animal  or  plant;  ANATOMY,  the  gross  structure  of  in- 
dividual parts,  or  organs;  HISTOLOGY,  the  microscopic 


FIG.   1.  —  The  chief  divisions  of  Biology. 

structure  of  organs,  or  tissues;  CYTOLOGY,  the  component 
elements  of  tissues,  or  cells,  and  the  physical  basis  of  life,  or 
protoplasm.  Similarly,  PHYSIOLOGY  investigates  the  activi- 
ties of  animals  and  plants,  the  functions  of  organs,  the 
properties  of  tissues,  the  phases  of  cell  life,  and  finally  the 
physico-chemical  characteristics  of  protoplasm.  So  much 
for  the  study  of  the  adult  individual  animal  or  plant  —  but 
this  is  not  all.  The  origin  and  development  of  the  individual, 


THE    SCOPE    OF   BIOLOGY  O 

GENETICS  and  EMBRYOLOGY;  and  the  origin  and  develop- 
ment of  species,  ORGANIC  EVOLUTION,  are  other  wide  fields, 
sciences  in  themselves,  which  must  be  approached  from  both 
the  structural  and  functional  aspect  if  any  real  advance  is 
to  be  made  toward  a  comprehensive  appreciation  of  life. 

(Fig.  1.) 

Thus,  just  as  the  various  physical  sciences  have  expanded 
and  become  specialized  until  they  are  beyond  the  grasp  of 
a  single  man,  so  biology  and  its  subdivisions,  or  the  BIO- 
LOGICAL SCIENCES,  are  now  distributed  among  many  special- 
ists. Although  specialization  results  in  a  narrowing  and 
isolating  of  the  fields  of  study,  as  deeper  levels  of  investiga- 
tion have  been  reached  in  all  the  sciences  there  has  been  a 
tendency  for  the  basic  phenomena  to  meet  on  the  common 
ground  of  the  fundamental  sciences,  physics  and  chemistry 
-for  in  the  last  analysis  the  biologist  must  assume  as  a 
working  hypothesis  that  the  properties  of  protoplasm  are  the 
resultant  of  the  properties  and  interrelationships  of  the 
chemical  elements  which  compose  it.  "In  one  direction, 
supported  by  chemistry  and  physics,  biology  becomes  bio- 
chemistry and  biophysics.  In  a  contrary  direction  it  forms 
a  connection  with  the  psychical  sciences  which  relate  to 
human  nature,  with  psychology  and  sociology,  with  ethics 
and  religion." 


CHAPTER  II 
THE  PHYSICAL  BASIS  OF  LIFE 

Science  never  destroys  wonder,  but  only  shifts  it,  higher 
and  deeper.  —  Thomson. 

THE  old  saying  that  the  materials  forming  /£he  human  bod}7 
change  completely  every  seven  years  is  a  taciFrecognition  that 
lifeless  material,  in  the  form  of  food,  is  gradually  transformed 
into  similar  living  matter  under  the  influence  of  the  body.  In- 
deed, just  as  a  geyser  retains  its  individuality  from  moment 
to  moment  though  it  is  at  no  two  instances  composed  of  the 
same  molecules  of  water  identically  placed,  so  the  living 
individual  is  a  focus  into  which  materials  enter,  play  a  part 
for  a  time,  .and  then  emerge  to  become  dissipated  in  the 
environment.  But  here  the  analogy  stops.  For  in  the 
living  organism  the  materials  which  enter  as  food,  endowed 
with  POTENTIAL  energy,  are  arranged  and  rearranged  until 
specific  molecular  aggregates  result,  which  in  turn  are  trans- 
formed into  integral  parts  of  the  organization  of  life  itself. 
However,  to  live  is  to  work,  and  to  work  means  expenditure 
-  the  transformation  of  the  potential  into  KINETIC  energy  - 
with  the  result  that  materials  in  relatively  simple  form  and 
largely  or  entirely  devoid  of  energy  are  returned  to  the 
realm  of  the  non-living. 

Thus  we  reach  a  fact  of  prime  importance:  so  far  as  we 
know,  living  matter  is  merely  ordinary  matter  which  has 
assumed,  for  the  time  being,  a  peculiar  condition  in  which  it 
displays  the  remarkable  series  of  phenomena  which  we 
recognize  as  LIFE. 

6 


THE    PHYSICAL   BASIS    OF    LIFE  7 

The  body  of  Man  in  common  with  that  of  all  animals  and 
plants  is  composed  of  living  and  non-living  matter  closely 
associated,  though  totally  distinct.  For  example,  the 
visible  parts  of  hair  and  nails,  a  large  part  of  bone  and  the 
liquid  part  of  blood  is  non-living  material.  But,  the  non- 
living is  not  confined  to  gross  structures,  for  the  dead  among 
the  living  is  still  revealed  until  the  resolving  power  of  the 
microscope  fails  us. 

A.    PROTOPLASM 

Although  there  is  a  continuous  stream  of  matter  and 
energy  flowing  through  the  living  individual,  nevertheless 
the  physical  and  chemical  study  of  living  matter  from  what- 
ever source  we  take  it  —  Mold  or  Elm,  Amoeba  or  Man  - 
reveals  a  remarkable  similarity  in  its  fundamental  factors, 
and  it  is  to  a  consideration  of  what  the  concept  PROTOPLASM 
holds  for  the  biologist  that  we  now  turn. 

As  the  finer  structure  of  animals  and  plants  came  within 
the  range  of  vision  through  improvements  in  microscope 
lenses,  it  was  gradually  recognized  that  the  ultimate  living  \ 
part  appeared  to  be  a  granular,  slime-like  material.  Thus 
Dujardin,  in  1835,  designated  as  sarcode  the  material  forming 
the  bodies  of  microscopic  animals.  Purkinje,  in  1840, 
named  the  formative  substance  of  the  developing  animal 
protoplasm,  and  compared  it  with  the  granular  material  of 
the  growing  region  of  certain  plants.  Six  years  later,  von 
Mohl  similarly  named  the  contents  of  the  finer  structural 
units  of  plants.  Confirmatory  observations  came  from  many 
sources  during  the  following  decade  and  culminated  in  the 
classical  studies  of  Max  Schultze  and  de  Bary  which  estab- 
lished the  full  physiological  significance  of  protoplasm  as  the 
essentially  similar,  fundamental,  living  material  of  both 
animals  and  plants.  This  reduction  of  all  life  phenomena 


8  FOUNDATIONS   Ci~  BIOLOGY 

to  a  common  denominator  was  the  final  justification  of  the 
prevision  of  the  earlier  workers  in  recognizing  a  life-science  — 
biology. 

Although  we  speak  of  a  common  '  physical  basis  of  life/  it 
is  of  paramount  importance  to  bear  in  mind  that  the  proto- 
plasm of  no  two  animals  or  plants  or,  indeed,  of  different 
parts  of  the  same  animal  or  plant  is  exactly  the  same. 
Identity  of  protoplasm  would  mean  identity  of  structure 
and  function  —  identity  of  life  itself.  The  concept  proto- 
plasm merely  emphasizes  that,  after  allowances  are  made 
for  all  the  variations,  we  still  have  the  similarities  far 
outnumbering  the  dissimilarities  in  the  'agent  of  vital 
manifestations.' 

The  physical  chemists  tell  us  that  matter  in  the  living 
state  represents  a  type  of  COLLOIDAL  CONDITION  of  matter 
known  as  an  emulsoid  which,  in  turn,  may  exist  either  as  a 
sol  —  the  apparently  homogeneous  liquid  state  of  living 
matter;  or  as  a  gel  —  the  apparently  amorphous  semi-solid 
state.  Protoplasmic  sols  appear,  as  a  rule,  homogeneous  be- 
cause of  the  exceedingly  small  size  of  the  molecular  aggre- 
gates which  form  them,  while  protoplasmic  gels  reveal  either 
a  homogeneous  or  heterogeneous  molar  structure  because  of 
the  relatively  large  particles  which  set  to  form  the  gel.  In 
other  words,  living  matter  holds  an  intermediate  position 
between  true  solids  and  true  liquids,  and  has  many  properties 
of  both,  as  well  as  many  peculiar  to  itself. 

But  this  leaves  the  reader  without  any  clear  conception 
of  the  appearance  of  protoplasm.  As  a  matter  of  fact  it  is 
as  difficult  to  describe  the  appearance  of,  as  it  is  to  define, 
protoplasm.  It  must  be  seen  under  the  microscope  to  be 
appreciated.  With  a  moderate  magnification,  protoplasm 
presents  a  fairly  characteristic  picture,  appearing  like  a 
translucent,  colorless,  viscid  fluid  containing  many  minute 


THE    PHYSICAL    BASIS    OF   LIFE  9 

granules  as  well  as  clear  spaces  or  vacuoles.  (Fig.  2.) 
If  it  is  examined  in  water  it  exhibits  no  tendency  to  mix  with 
the  surrounding  medium,  though  investigations  show  that 
osmotic  interchanges  are  constantly  going  on.  For  this 
reason  it  is  impossible  to  consider  protoplasm  except  in 
connection  with  its  surroundings  whatever  they  may  be  — 


FIG.  2.  —  A  simple  animal  (Amoeba  proteus)  which  consists  ot  a  single  unit  mass 
of  protoplasm  (highly  magnified).  1,  nucleus;  2,  contractile  vacuole;  3,  pseudopodia; 
4,  food  material  in  process  of  digestion  (food  vacuole);  5,  sand  particle  or  other  indi- 
gestible inclusion.  (From  Shipley  and  McBride,  after  Gruber.) 

variations  in  its  environment  and  variations  in  its  activities 
being  reflected  directly  or  indirectly  in  its  appearance. 
Under  the  highest  magnifications,  not  only  does  the  finer 
structure  of  protoplasm  differ  in  various  specimens,  but  also 
in  the  same  living  unit  mass  under  slightly  different  physi- 
ological conditions.  At  one  time  it  presents  the  appearance 
of  a  fairly  definite  net-like  structure,  or  reliculum,  the  meshes 
of  which  enclose  a  more  fluid  substance;  at  another,  a  frothy 


10 


FOUNDATIONS    OF   BIOLOGY 


appearance  in  which  the  alveoli,  or  'bubbles, '  represent  a  more 
liquid  substance  emulsified  in  a  less  liquid  medium.  Again, 
at  other  times,  the  denser  portion  seems  to  take  the  form  of 

minute  rods,  or  fibers,  distributed  in 
a  somewhat  fluid  matrix.  (Fig.  3.) 
These  appearances  have  given 
rise  to  various  theories  which  em- 
phasize one  or  another  as  the 
universal  formula  for  the  physical 
structure  of  protoplasm,  from 
which  the  other  appearances  are 
merely  secondarily  derived.  But 
the  trend  of  recent  work  has  been 
to  indicate  that  although  the  gen- 
eral similarity  of  protoplasmic  ac- 
tivity, wherever  we  find  it,  might 
lead  us  to  expect  to  find  also  a 
visible  fundamental  structural 
basis,  such  does  not  exist  within 
the  range  of  magnifications  at  our 
command.  Reticular,  alveolar,  and 
fibrillar  structures  which  our  mi- 
croscopes reveal  are,  as  it  were, 
merely  surface  ripples  from^  underlying  physico-chemical 
changes  which,  thus  far,  have  proved  unfathomable. 

B.   CHARACTERISTICS  OF  LIVING  MATTER 

Since  the  phenomena  of  life  are  without  exception  the 
results  of  protoplasmic  activity,  it  is  obvious  that  we  must 
look  to  protoplasm  for  the  primary  attributes  of  living 
matter.  The  properties  which  are  absolutely  diagnostic  of 
living  matter  are  its  l  chemical  composition, ^metabolism 
including  the  power  of  waste  and  repair,  growth  by  intus- 


FIG.  3.  —  Alveolar  appearance 
of  the  protoplasm  of  a  cell  from  the 
skin  (epidermis)  of  an  Earthworm. 
(From  Verworn,  after  Butschli.) 


THE    PHYSICAL   BASIS    OF   LIFE 

^ 

susception,  the  power  of  reproduction,  the  power  of  adapta- 
tion, and  specific  form  and  organization. 

1.   Chemical  Composition 

It  is  impossible  to  make  an  analysis  of  living  matter 
because  the  disturbance  of  its  molecular  organization  by 
chemical  reagents  kills  it.  Therefore  our  knowledge  of  its 
chemical  composition  has  of  necessity  been  derived  from  a 
study  of  dead  protoplasm.  However,  since  in  the  trans- 
formation from  the  living  to  the  non-living  state  there  is 
clearly  no  loss  of  weight,  it  follows  that  the  complete  material 
basis  of  life  is  still  present  for  examination.  In  other  words, 
the  death  of  protoplasm  is  a  result  of  disorganization. 

Chemical  analysis  of  protoplasm  shows  that  it  invariably 
comprises  the  elements  carbon,  oxygen,  hydrogen,  nitrogen, 
sulfur,  and  phosphorus;  and  usually  also  chlorine",  potassium, 
sodium,  magnesium',  calcium,  and  iron.  Occasionally  a  num- 
ber of  other  elements  are  found  normally  in  the  protoplasm 
of  certain  parts  of  various  species  of  animals  and  plants. 

The  average  composition  of  the  human  body  is  about  as 

follows : 

Oxygen 65.00% 

Carbon 18.00 

Hydrogen 10.00 

Nitrogen 3.00 

Calcium 2.00 

Phosphorus. . 1.00 

Potassium 0.35 

Sulfur... 0.25 

Sodium 0.15 

Chlorine.. 0.15 

Magnesium. 0.05 

Iron 0.004 

Iodine traces 

Fluorine traces 

Silicon..  traces 


12  FOUNDATIONS   OF   BIOLOGY 

At  first  glance  there  is  nothing  very  striking  about  this 
list  of  elements.  They  are  all  common  ones  with  which  the 
chemist  is  familiar  in  the  non-living  world.  But  it  is  the 
combination  of  the  elements  which  is  significant,  and  this 
results  from  the  capacity  of  carbon,  hydrogen,  and  oxygen, 
or  carbon  and  hydrogen  together,  to  form  the  numerous 
complex  compounds  which  in  turn  supply  the  basis  for  inti- 
mate associations  with  other  elements.  As  a  matter  of  fact, 
the  bulk  of  protoplasm  is  composed  of  carbon,  oxygen, 
hydrogen,  and  nitrogen  associated  with  each  other  in  an 
apparently  infinite  series  of  relationships,  in  which  the 
carbon  seems  to  play  the  leading  role.  Some  of  these  com- 
pounds are  relatively  simple,  such  as  water  (H2O)  which  is 
quantitatively  the  most  important  constituent  of  all  proto- 
plasm, but  the  majority  consist  of  elaborate  atomic  arrange- 
ments and  not  a  few  represent  molecular  complexes  of  hun- 
dreds and  even  thousands  of  atoms. 

The  compounds  of  carbon  which  are  characteristic  of 
protoplasm  fall  into  three  chief  groups:  proteins,  carbohy- 
drates, and  fats. 

PROTEINS  invariably  consist  of  the  elements  carbon, 
oxygen,  hydrogen,  nitrogen,  and  sulfur,  and  frequently 
phosphorus  and  iron;  Examples  are  albumin  of  the  white  of 
egg,  casein  of  milk,  gluten  of  cereals,  and  myosin  of  lean 
meat.  The  nitrogen  particularly  distinguishes  proteins 
from  the  other  compounds  of  the  living  complex  and,  as  we 
shall  see  later  when  considering  the  chemical  processes  in 
animals  and  plants,  is  largely  responsible  for  their  command- 
ing position  as  "the  chemical  nucleus  or  pivot  around  which 
revolve  a  multitude  of  reactions  characteristic  of  biological 
phenomena."  Study  of  the  relationship  of  nitrogen  to  the 
other  chemical  elements  of  proteins  long  since  revealed  the 
fact  that  the  protein  molecule  is  a  huge  complex  of  linked 


a  -=?•  K  ,  v/  '-* 

' 


THE    PHYSICAL   BASIS    OF   LIFE       ^H   -{   13 


AMINO  ACIDS  —  an  amino  acid  being  an  organic  acid  in 
which  one  hydrogen  atom  is  replaced  by  the  amino  group, 
NH2.  But  at  the  present  time  it  is  becoming  increasingly 
patent  that  the  amino  acids  are,  as  it  were,  the  nitrogenous 
units  with  which  organisms  deal  physiologically,  rather  than 
the  proteins  themselves.  An  animal,  for  example,  with 
various  proteins  available  in  its  food,  chemically  disrupts 
these  into  their  amino  acid  constituents,  and  then  takes  an 
amino  acid  here  and  another  there  and  synthesizes  the 
specific  proteins  it  demands.  And  further,  if  individual 
amino  acids  are  supplied,  the  animal  employs  them.  So  it 
seems  highly  probable  that  the  specific  structure  of  an  or- 
ganism depends  upon  the  chemical  specificity  of  its  proteins. 

Although  the  presence  of  proteins  and  the  power  of  form- 
ing them  is  the  chief  diagnostic  chemical  characteristic  of 
living  matter,  at  the  present  stage  of  our  knowledge  it  is 
impossible  to  define  proteins  satisfactorily  on  the  basis  of 
chemical  or  physiological  properties.  The  most  we  can  say 
is  that  the  biochemist  describes  proteins  as  "huge  molecules, 
complex  in  structure,  labile  in  character,  and  therefore  prone 
to  chemical  change"  —and  the  latter  characteristic  un- 
doubtedly is  closely  associated  with  the  perennial  plasticity 
and  responsiveness  of  the  protoplasmic  system  itself. 

CARBOHYDRATES  consist  of  various  combinations  of 
carbon,  hydrogen,  and  oxygen,  the  latter  elements  invariably 
being  present  in  the  proportion  found  in  water  (H20). 
Though  more  simple  in  chemical  structure  than  proteins, 
they  range  in  complexity  from  the  simple  sugars,  or  monosac- 
charids,  such  as  glucose  and  fructose,  to  polysaccharids  such 
as  dextrins,  starches,  and  cellulose. 

FATS  are  composed  of  the  same  elements  as  the  carbo- 
hydrates, but  in  quite  different  arrangements.  The  propor- 
tion of  oxygen  is  always  less,  and  therefore  they  are  more 


14  FOUNDATIONS    OF   BIOLOGY 

oxidizable  and  richer  in  potential  energy.  Fats  represent 
a  synthesis  of  an  acid  (fatty  acid)  and  glycerine.  Examples 
are  butter  and  all  oils  of  plant  or  animal  origin. 

Thus  proteins,  carbohydrates,  and  fats  represent  large 
classes  of  substances  which  are  distinctly  characteristic  of 
living  matter,  not  being  found  in  nature  except  as  the  result 
of  protoplasmic  activity;  although  biochemists  now  can 
artificially  synthesize  certain  fats  and  carbohydrates  as  well 
as  the  amino  acid  constituents  of  some  proteins.  Proteins 
undoubtedly  play  the  most  important  part  in  the  organiza- 
tion of  protoplasm,  while  the  carbohydrates  and  fats  contrib- 
ute largely  to  the  supply  of  available  energy.  However,  it  is 
impossible  to  draw  a  hard  and  fast  distinction  in  regard  to 
their  respective  contributions  because,  for  example,  as  we 
shall  see  later,  carbohydrates  form  the  foundation  upon 
which  proteins  are  synthesized  by  green  plants. 

Proteins,  carbohydrates,  and  fats  are  frequently  referred 
to  as  the  foodstuffs,  but  it  will  be  recognized  that  while,  in  a 
way,  they  constitute  the  chief  groups,  all  the  constituents 
of  protoplasm  must  be  available.  Accordingly,  inorganic 
salts,  water,  and  free  oxygen  are  really  foodstuffs.  Further- 
more, recent  investigation  has  disclosed  another  class  of 
organic  substances  which  are  absolutely  necessary  for  the 
constructive  phases  of  protoplasmic  activity.  These  are 
termed  VITAMINES  and  must  be  classified  as  accessory  food 
substances,  although  as  yet  little  is  known  in  regard  .to  their 
chemical  structure  or  mode  of  action.  And  then,  finally,  on 
the  border  line  of  food  substances  may  be  mentioned  a  great 
group  of  organic  catalyzers,  called  ENZYMES,  which  play  a 
major  role  in  metabolism.  But,  when  all  is  said,  our  knowl- 
edge of  the  chemical  complexities  of  protoplasm  affords  no 
adequate  conception  of  how  they  are  related  to  the  phe- 
nomena of  life.  This  is  beyond  present-day  biology. 


THE    PHYSICAL    BASIS    OF    LIFE  15 

2.   Metabolism 

We  have  emphasized  that  living  matter  is  continually 
changing,  and  this  fundamental  fact  is  reflected  in  nearly  all 
attempts  to  define  life.  Aristotle  described  life  as  "the 
assemblage  of  operations  of  nutrition,  growth,  and  destruc- 
tion"; deBlainville,  as  a  "  twofold  internal  movement  of 
composition  and  decomposition";  and  Spencer,  as  "the 
continuous  adjustment  of  internal  relations  to  external 
relations." 

This  interaction  consists  of  chemical  and  physical  pro- 
cesses in  which  combustion  or  oxidation  plays  the  chief  role. 
Lavoisier  and  Laplace  in  1780  showed  that  animal  heat  results 
from  a  slow  burning  of  the  materials  of  the  body,  involv- 
ing the  consumption  of  oxygen  and  the  liberation  of  carbon 
dioxide;  and  further,  that  for  a  given  consumption  of  oxygen 
and  liberation  of  carbon  dioxide,  about  the  same  amount  of 
heat  is  produced  by  an  animal  as  by  a  burning  candle.  This 
was  an  important  discovery,  because  it  went  far  toward 
establishing  the  fact  that  at  least  certain  characteristic  vital 
phenomena  are  amenable  to  the  laws  which  hold  in  the  non- 
living world. 

But  the  processes  involved  in  life  are  not  so  simple  as 
perhaps  might  be  imagined  from  the  results  just  mentioned. 
Heat  represents  but  one  of  the  many  energy  transformations 
within  the  organism.  Indeed  the  living  organism,  like  a 
steam  engine,  is  a  machine  for  transforming  energy  —  trans- 
forming the  potential  energy  stored  in  chemical  complexes 
of  its  own  substance  into  the  various  vital  processes  of  living 
-  into  work  performed.  In  these  processes  many  complex 
substances  rich  in  potential  energy,  which  have  entered 
as  food  and  have  in  whole  or  part  added  to  the  protoplasmic 
complex,  are  reduced  to  simpler  and  simpler  conditions  and 


16  FOUNDATIONS   OF   BIOLOGY 

finally,  with  their  energy  content  nearly  or  entirely  ex- 
hausted, are  eliminated  as  EXCRETIONS.  This  continual 
waste  must,  if  life  is  to  persist,  be  counterbalanced  by  a 
proportionate  intake  of  food  in  order  to  renew  the  supply  of 
energy  and  afford  the  materials  which,  after  preliminary 
changes,  are  made  into  an  integral  part  of  the  living  organ- 
ism. Thus  in  living  the  animal  or  plant  is  partially  consum- 
ing and  rebuilding  itself  continually.  This  dual  process  is 
METABOLISM.  When  constructive  metabolism.  ANABOLISM^ 
keeps  pace  with  destructive  metabolism.  KATABOLISM^  the 
individual  remains  essentially  unchanged  and  this  is  the 
normal  condition  of  adult  life.  During  youth  the  anabolic 
phases  are  in  the  ascendency  and  growth  occurs,  while  old 
age  is  characterized  by  a  predominance  of  katabolic  processes. 

3.   Growth 

The  results  of  metabolism  force  themselves  upon  our 
attention  chiefly  as  growth,  or  permanent  increase  in 
the  size  of  the  individual.  As  a  rule  growth  in  plants  con- 
tinues more  or  less  rapidly  throughout  life,  while  in 
animals  it  is  confined  mainly  to  the  early  part  of  the 
individual's  existence,  or  youth.  Indeed,  at  birth  a  child 
is  about  a  billion  times  larger  than  the  egg  from  which  it 
has  developed. 

Growth  means  that  the  organism  makes  over  the  materials 
which  it  receives  in  the  form  of  food  from  its  environment 
and  fits  them  into  the  protoplasmic  organization  here  and 
there  throughout  as  needed.  This  method  'of  addition  of 
materials,  which  is  termed  growth  by  INTUSSUSCEPTION,  is 
highly  characteristic  of  life.  When  growth  occurs  in  the 
non-living  world,  it  is  typically  by  accretion;  as,  for  example, 
in  crystals  where  new  material  of  the  same  kind  is  superim- 
posed upon  the  surface.  But  protoplasm,  with  materials 


THE    PHYSICAL   BASIS    OF   LIFE  17 

and  energy  taken  from  its  environment,  constructs  more 
protoplasm  and,  if  the  available  materials  are  adequate,  the 
specifically  organized  living  substance  tends  to  increase 
indefinitely.  Thus  it  is  not  only  the  method  of  growth 
which  is  diagnostic  of  animals  and  plants,  but  also  the  fact 
that  when  the  individual  body  has  reached  a  certain  phys- 
iological balance,  or  maturity,  in  which  it  ceases  to  increase 
in  size,  under  normal  conditions  it  expresses  the  inherent 
growth  power  of  living  matter  by  setting  free  certain  living 
units,  which  go  through  a  cycle  of  growth  phenomena  that 
result  in  re-productions  of  the  parent  individual. 

4.   Reproduction 

So  far  as  is  known,  living  matter  never  arises  except  under 
the  direct  influence  of  preexisting  living  matter.  We  have 
seen  that  this  transformation  is  continually  going  on  in  the 
constructive  phase  of  metabolism  in  the  animal  or  plant, 
and  brings  about  repair  and  growth  of  the  individual; 
but  it  is  in  reproduction  that  what  may  be  termed  the  over-  . 
growth  of  the  individual  results  in  the  production  of  a  new 
one.  A  larger  or  smaller  part  of  the  parent  generation  is 
detached  and  becomes  the  new  generation,  so  that  in  ultimate 
analysis  reproduction  is  division.  This  is  a  highly  unique 
characteristic  of  living  things  which  provides  for  the  con- 
tinuation of  the  race. 

5.   Adaptation 

The  discussion  of  metabolism  has  emphasized  the  close 
interrelationship  between  the  living  complex  and  its  sur- 
roundings, and  the  dependence  of  life  upon  the  interplay 
and  interchange  between  protoplasm  and  its  environment. 
As  a  matter  of  fact  the  plant  or  animal  retains  its  individual- 
ity —  lives  —  solely  by  its  powers  of  developing  and  main- 


18  FOUNDATIONS   OF   BIOLOGY 

taining  exquisite  adjustments  to  its  surroundings.  This 
results  from  the  IRRITABILITY  of  living  substance :  its  inherent 
capacity  of  reacting  to  environmental  changes  by  changes 
in  the  equilibrium  of  its  matter  and  energy.  The  inciting 
Changes,  known  as  STIMULI,  may  be  chemical,  electrical, 
/thermal,  photic,  or  mechanical,  but  the  nature  of  the  response 
is  determined  rather  by  the  fundamental  character  of  the 
protoplasmic  system  itself  than  by  the  nature  of  the  stimulus. 
Muscle  protoplasm  contracts  however  it  is  stimulated.  The 
reaction  of  living  matter  by  virtue  of  its  intrinsic  irritability 
implies  not  only  response  to  a  stimulus  but  also  conduction 
so  that  the  protoplasmic  system  as  a  whole  is  directly  or 
indirectly  influenced.  It  responds  as  a  coordinated  unit  — 
an  individual.  It  adapts  itself  structurally  and  functionally 
to  the  exigencies  of  its  existence.  This  power  of  adaptation, 
as  exhibited  in  active  adjustment  between  internal  and 
external  relations,  overshadows  every  manifestation  of  life 
and  contributes,  more  than  any  other  factor,  to  the  "  enor- 
mous gap  that  separates  even  the  lowest  forms  of  life  from 
the  inorganic  world." 

6.   Organization 

Finally,  adaptation  implies  that  living  things  are  not 
homogeneous,  but  exhibit  reciprocal  structural  and  physio- 
logical organization.  Accordingly  animals  and  plants  are 
referred  to  as  organisms.  Indeed  a  major  part  of  the  present 
volume  is  devoted  to  the  organization  of  organisms. 

The  characteristics  which  we  have  described  —  chemical 
composition,  metabolism  including  waste  and  repair,  growth 
by  intussusception,  reproduction,  adaptation,  and  specific 
organization  —  individually  and  collectively  are  diagnostic 
of  living  matter.  It  is  possible,  to  be  sure,  to  take  exception 
to  one  or  another;  e.g.,  to  say  that  growth  by  intussusception 


THE    PHYSICAL   BASIS    OF   LIFE  19 

occurs  in  non-living  things  when  a  salt  is  dissolved  in  water; 
but  such  formal  objections  only  emphasize  the  unique  condi- 
tions which  obtain  in  life. 

The  reader  may  be  surprised  to  note  that  the  power  of 
movement  has  not  been  mentioned  as  a  characteristic  of  life, 
but  a  moment's  thought  will  make  it  apparent  that  visible 
movement  is  not  confined  to  living  matter.  Though  this  is 
so,  movement  is  one  of  the  most  obvious  manifestations  of 
life  and  depends,  of  course,  in  every  instance,  upon  molar 
changes  resulting  from  tumultuous  ultramicroscopic  chemical 
changes  of  protoplasm  itself. 

And  it  is  to  these  changes  that,  in  the  last  analysis,  we  must 
turn  for  the  energy  which  brings  about  the  visible  movements 
in  animals  and  plants,  such  as  the  contraction  of  the  muscles 
of  animals,  the  streaming  movement  (amoeboid  movement) 
of  the  simple  animals  known  as  Amoebae,  the  rotation  and 
circulation  of  the  protoplasm  in  certain  of  the  living  units  of 
plants  and,  finally,  the  lashing  of  threads  of  cytoplasm  (cilia) 
which  not  only  enables  many  a  tiny  plant  and  animal  to  swim, 
but  also  aids  in  numerous  ways  in  certain  parts  of  the 
bodies  of  higher  organisms.  The  phenomena  of  life  are  quite 
generally  expressed  in  visible  movements,  but  the  latter  are 
not  peculiar  to  living  things. 

In  our  discussion  thus  far  we  have  endeavored  to  describe 
the  characteristics  of  matter  in  the  living  state  on  the  basis 
of  the  fundamental  vehicle  of  life  manifestations  —  proto- 
plasm. We  have  not  attempted  formally  to  define  'life'  or 
'  protoplasm '  because  they  are  so  unique  that  it  is  impossible 
to  resort  to  the  lexicographer's  trick  of  comparing  them  with 
something  else;  and  because  the  expressions  'protoplasm' 
and  'life'  are  abstractions;  one  indicating  that  all  individual 
animals  and  plants  have  to  a  large  extent  a  common  organi- 
zational foundation,  and  the  other  that  they  exhibit  certain 


20  FOUNDATIONS    OF   BIOLOGY 

characteristic  actions  and  reactions.  The  living  organism 
is  a  microcosm  which  exhibits  a  permanence  and  continuity 
of  individuality  correlated  with  specific  behavior,  and  this  it 
transmits  to  other  matter  which  it  makes  a  part  of  itself, 
and  to  its  offspring  in  reproduction. 


CHAPTER  III 


ORGANIZATIONAL  UNITS  OF  PLANTS 
AND  ANIMALS 

Over  the  structure  of  the  chemical  molecule  rises  the  struc- 
ture of  the  living  substance  as  a  broader  and  higher  kind  of 
organization.  Over  the  structure  of  the  cell  rises  again  the 
structure  of  plants  and  animals,  which  exhibit  the  yet  more 
complicated,  elaborate  combinations  of  millions  and  billions 
of  cells  coordinated  and  differentiated  in  the  most  extremely 
different  ways.  — -Hertwig. 

/SINCE  living  matter  is  only  known  to  us  in  the  form  of 
individual   animals   and   plants,    individuals   are   the   only 


FIG.  4.  —  Cells,  highly  magnified,  from  the  surface  of  the  Frog's  skin  (A), 
and  a  plant  loaf  (B) . 

realities  in  living  nature,  and  we  turn  now  to  a  consider- 
ation of  the  organization  of  the  individual. 

A  thin  slice  of  material  from  the  surface  of  the  skin  of  a  Frog 
or  the  leaf  of  a  Buttercup  shows  under  the  microscope  the 
same  general  structure.  Each  appears  to  be  a  mosaic  of  in- 
numerable small  bodies,  no  two  of  which  are  exactly  alike  even 
in  the  same  piece,  though  all  are  similar  enough  to  be  one  and 

21 


22 


FOUNDATIONS   OF   BIOLOGY 


the  same  type  of  unit.     And  if  we  extend  our  study  to  other 
parts  of  the  Buttercup  or  the  Frog  or,  indeed,  to  any  part  of 


FIG.  5.  —  Vertical  section  (highly  magnified)  of  a  leaf  to  show  its  cellu- 
lar structure,  a,  guard  cells,  at  opening  (stoma)  through  epidermis; 
b,  cells  containing  chlorophyll;  c,  upper  and  lower  epidermal  cells. 
(From  Abbott,  after  Bailey.) 

any  familiar  plant  or  animal,  we  find  essentially  similar  units 
of  structure  in  every  case.     In  fact,  the  bodies  of  all  living 


FIG.  6.  —  Transverse  section  (highly  magnified)  of  a  simple  animal 
(Hydra)  to  show  the  cellular  structure.  Outer  layer,  ectoderm;  inner 
layer,  endoderm;  central  cavity,  enteron.  (After  Shipley  and 
McBride.) 

things  either  consist  of  a  single  organic  unit  or  are  congeries 
of  millions  of  essentially  similar  units  called  CELLS.     (Fig.  4.) 


ORGANIZATIONAL   UNITS  23 

Such  being  the  case  we  reach  another  great  generalization : 
all  organisms  have  the  same  elementary  structure,  just  as 
we  have  seen  that  all  organisms  are  composed  of  a  similar 
fundamental  life-stuff,  protoplasm.  Therefore  a  cell  may 
be  described  as  a  small  mass  of  protoplasm,  either  living 
apart  as  a  microscopic  plant  or  animal,  or  forming  a  building 
block,  as  it  were,  in  the  architecture  of  one  of  the  higher 
organisms.  Indeed,  organisms  are  organisms  because  of 
specific  local  differentiations  in  the  living  material  —  this 
differentiation  being  possible  largely  because  the  protoplasm 
is  disposed  in  microscopic  unit  masses,  or  cells.  (Figs.  5,  6.) 

The  appreciation  of  this  dual  similarity  —  protoplasmic 
basis  and  cellular  organization  —  of  all  living  things,  which 
was  finally  attained  about  the  middle  of  the  last  century, 
firmly  established  the  fact  that  all  living  nature  is  one,  — 
the  corner  stone  of  modern  biology. 


A.   THE  CELL 

Having  taken  a  general  survey  of  the  building  materials 
of  living  nature,  microscopic  unit  masses  of  protoplasm, 
termed  cells,  we  are  now  in  a  position  to  consider  in  some 
detail  the  structure  of  a  typical  cell.  With  the  diversity 
of  gross  structure  of  animals  and  plants  in  mind,  one  is  not 
surprised  that  there  are  considerable,  even  great,  variations 
in  their  component  elements.  In  fact  the  characteristics  of 
an  organism  or  part  of  an  organism  are  determined  by  those 
of  its  cells.  But  there  are  certain  generalized  cell  characters 
which  are  common  to  all  cells  —  by  virtue  of  which  they 
are  cells  —  and  it  is  important  to  emphasize  these. 

In  its  simplest  form  a  cell  is  a  small  spherical  mass  of 
protoplasm.  Such  are  the  eggs  of  various  animals  and  the 
complete  body  of  some  of  the  lowest  plants  and  animals. 


24  FOUNDATIONS   OF   BIOLOGY 

Cells  forming  the  units  of  multicellular  organisms,  however, 
frequently  exhibit  more  or  less  hexagonal  surfaces  on  account 
of  stresses  and  strains  incident  to  their  position  among  other 
cells,  while  specializations  and  differentiations,  for  one  purpose 
or  another,  produce  forms  which  are  characteristic  of  different 
parts  of  the  organism,  as,  for  example,  the  long  spindle- 
shaped  cells  of  smooth  muscle,  and  the  widejv_J^ranching 
cells  of  parts  of  the  brains  of  animals.  Broadly  speaking, 
the  greater  diversity  of  cell  form  is  found  in  animals,  while 
in  plants,  owing  to  the  more  general  presence  of  rigid  cell 
walls  about  the  protoplasm,  the  units  more  frequently  present 
symmetrical,  angular  outlines.  (Figs.  7,  42.) 

The  term  cell  is  a  relic  of  the  time  when  the  cell  wall  was 
regarded  as  the  more  important  part,  and  its  protoplasmic 
contents,  if  observed  at  all,  were  considered  as  only  of  second- 
ary importance,  if  not  a  waste  product.  Now  we  recognize 
many  cells  which  are  merely  naked  masses  of  protoplasm, 
such  as  certain  types  of  blood  cells.  In  other  words,  the 
protoplasm  is  the  essential  living  part  —  the  cell  wall 
frequently  being  a  non-living  accessory  which  more  or  less 
sharply  delineates  one  unit  mass  of  protoplasm  from  another 
and  lends  rigidity  and  form  to  the  group  of  cells  as  a  whole. 

1.  Cytoplasm 

The  protoplasm  of  all  typical  cells  is  differentiated  into 
two  parts:  the  CYTOPLASM,  or  general  groundwork  which 
makes  up  the  bulk  of  the  cell;  and  the  NUCLEUS,  a  restricted, 
clearly  defined  area,  usually  situated  near  the  center  of  the 
cytoplasmic  mass. 

The  cytoplasm  may  be  considered  the  more  generalized 
protoplasm  of  the  cell,  and  its  appearance  and  other  character- 
istics are  those  which  have  been  outlined  in  our  discussion  of 
protoplasm.  With  that  in  mind,  for  the  sake  of  definiteness, 


ORGANIZATIONAL   UNITS 
A  C 


25 


FIG.  7.  — Various  types  of  cells,  highly  magnified:  A,  female  germ  cell,  egg  of  a  Cat. 
B,  male  germ  cell,  sperm  of  a  Snake.  C,  three  ciliated  epithelial  cells  from  the  digestive 
tract  of  a  Clam .  D,  supporting  tissue  (cartilage)  of  a  Squid.  E,  voluntary  (striated)  mus- 
cle fiber  from  an  Insect.  F,  involuntary  (smooth)  muscle  fibers  from  the  bladder  of  a  Calf. 
6,  nerve  cell  from  the  brain  of  Man.  (From  Hegner,  after  Dahlgren  and  Kepner.) 


26 


FOUNDATIONS   OF   BIOLOGY 


we  may  consider  its  basis  as  consisting  of  a  meshwork,  com- 
posed of  innumerable,  minute  granules  which  permeate  an 
apparently  homogeneous  hyaline  ground-substance.  Dis- 
tributed throughout  the  cytoplasm  are  usually  various  lifeless 
inclusions  such  as  granules  of  food,  droplets  of  water  or  oil, 
vacuoles  of  cell  sap,  crystals,  etc.,  representing  materials 


FIG.  8.  — Diagram  of  a  cell,  a-d,  nucleus;  a,  nucleolus;  b,  chromatin 
'network';  c,  linin  meshwork;  d,  karyosome,  or  chromatin  knot;  e,  meta- 
plasmic  inclusions  in  cytoplasmic  meshwork;  /,  vacuole;  g,  plastida  in 
cytoplasm;  h,  centrosome,  recently  divided.  (From  Wilson.) 

which  are  to  be,  or  have  been,  a  part  of  the  living  complex, 
or  are  by-products  of  the  vital  processes.  This  passive 
material  is  frequently  referred  to  as  METAPLASM,  but  it  is 
quite  evident  that  such  a  term  stands  for  no  essential  mor- 
phological part  of  the  cell,  and  we  have  no  absolute  criterion 
to  distinguish  between  some  granules  which  are  regarded  as 


. 


ORGANIZATIONAL    UNITS  27 

metaplasmic  in  nature  and  others  which  are  ordinarily  con- 
sidered active  elements  of  the  cytoplasm.  Specialized  living 
cytoplasmic  bodies,  known  as  PLASTIDS,  are  sometimes  also 
present.  Finally,  within  the  cytoplasm  in  the  vicinity  of  the 
nucleus,  there  is  frequently  visible  a  differentiated  area  con- 
taining a  CENTROSOME,  an  important  cell  organ  which  is  es- 
pecially active  during  cell  reproduction.  (Fig.  8.) 

The  cytoplasm,  since  it  forms  the  general  groundwork,  is 
that  part  of  the  cell  which  comes  most  closely  into  relations 
with  the  environment,  and  accordingly  near  the  surface  it  is 
frequently  modified  somewhat  in  texture  and  consistency  so 
that  an  outer  region,  or  ECTOPLASM,  may  be  distinguished 
from  an  inner,  or  ENDOPLASM.  Again,  the  cell  may  form 
about  itself  a  definite  membrane  or  a  heavy  cell  wall.  Nearly 
all  gradations  exist  between  highly  differentiated  cytoplasm 
(ectoplasm)  and  definite  membranes  and  cell  walls.  The 
ectoplasm  is  certainly  a  part  of  the  living  protoplasm,  while 
certain  types  of  membranes  and  cell  walls  must  be  regarded 
as  non-living,  though  in  many  cases  they  are  direct  trans- 
formations of  the  living  materials  which  grow  and  play  an 
important,  indeed  a  crucial,  part  in  controlling  directly  or 
indirectly  the  flow  of  matter  and  energy  to  and  from  the  cell 
and  its  surroundings. 

2.  Nucleus 

Within  the  cytoplasmic  mass  there  is  a  restricted  area 
of  clearly  differentiated  material,  which  typically  has  a 
rounded  form,  bounded  by  a  membrane,  so  that  it  appears 
as  a  definite  body  of  protoplasm  called  the  nucleus.  The 
structural  basis  of  the  nucleus  appears  to  be  essentially 
similar  to  that  of  the  cytoplasm  —  the  so-called  LININ  mesh- 
work  and  KARYOLYMPH  being  comparable  respectively  to  the 
granular  meshwork  and  hyaline  ground  substance  of  the 


28  FOUNDATIONS   OF   BIOLOGY 

cytoplasm.  But  superimposed  upon  this,  as  it  were,  is  the 
highly  characteristic  nuclear  material,  or  CHROMATIN,  which 
takes  various  forms  during  different  phases  of  cell  activity 
but  generally,  in  a  'resting'  cell,  gives  the  appearance  of  a 
network  of  tiny  granules  with  one  or  more  dense  'knots'  of 
chromatin  (KARYOSOMES)  .  Later  we  shall  describe  some  of 
the  important  changes  in  chromatin  arrangement,  but  it  is 
sufficient  at  this  time  to  emphasize  that  the  nucleus  is  a 
differentiated  area  of  the  cell  protoplasm  which  is  the  arena 
of  the  chromatin.  Frequently  there  is  a  conspicuous  round 
achromatic  body  within  the  nucleus  known  as  the  NUCLEOLUS. 
Cytoplasm  and  nucleus,  looked  at  from  the  functional  view- 
point, represent  a  physiological  division  of  labor  within  the 
confines  of  the  cell.  Experiments  have  shown  that  they  are 
mutually  necessary  for  cell  life;  the  removal  of  the  nucleus 
putting  an  end  to  anabolic  processes  —  assimilation,  repair, 
and  growth  —  and  thus  leading  rapidly  to  death.  Accord- 
ingly the  nucleus  may  be  considered  as  the  center  of  the 
synthetic  activities  of  the  cell,  and  the  cytoplasm,  if  not  as 
the  area  in  which  destructive  processes  are  chiefly  involved, 
at  least  as  a  neutral  region  which  translocates  material 
toward  and  away  from  the  nucleus.  All  the  evidence  points 
to  the  nucleus  as  the  ''controlling  center  in  cell  activity,  and 
hence  a  primary  factor  in  growth,  development,  and  trans- 
mission of  specific  qualities  from  cell  to  cell,  and  so  from  one 
generation  to  another." 

B.   ORIGIN  OF  CELLS 

With  regard  to  the  origin  of  life  on  the  Earth  absolutely 
nothing  is  known.  But  all  the  evidence  at  hand  tends  to 
show  that,  at  the  present  time  at  least,  living  matter  never 
arises  except  under  the  influence  of  preexisting  living  matter. 
That  is,  protoplasm  grows  —  cells  grow  and,  having  attained 


ORGANIZATIONAL   UNITS  29 

a  certain  size,  reproduce  by  dividing  into  two  more  or  less 
equal  parts.  The  process  of  cell  division  involves  the  divi- 
sion of  both  cytoplasm  and  nucleus,  and  therefore  we  must 
enlarge  our  conception  of  a  cell  as  a  small  mass  of  protoplasm 
differentiated  into  cytoplasm  and  nucleus,  by  adding  that 
both  cytoplasm  and  nucleus  arise  through  the  division  of  the 
corresponding  elements  of  a  preexisting  cell. 

We  shall  later  have  occasion  to  make  a  study  of  the  details 
of  cell  division,  known  as  MITOSIS,  but  from  what  has  been 
said  it  must  occur  to  the  reader  that,  since  cells  arise  only 
by  division,  those  of  the  present  day,  whether  complete 
free  living  organisms  or  units  composing  the  bodies  of  higher 
animals  and  plants,  are  actually  lineal  descendants  in  un- 
broken series  from  the  beginning  of  life  on  the  Earth.  The 
bond  of  discontinuity  between  parent  and  offspring  is  typi- 
cally a  single  cell  division,  (Fig.  123.) 


CHAPTER  IV 
METABOLISM   OF  GREEN  PLANTS 

Matter  and  force  are  the  two  names  of  the  one  Artist  who 
fashions  the  living  as  well  as  the  lifeless.  —  Huxley. 

IT  has  been  emphasized  that  life  is  only  known  to  us  in  the 
form  of  individuals,  and  we  turn  now  to  concrete  examples 
of  unicellular  plants  and  animals  which  present,  in  relatively 
simple  form  within  the  confines  of  a  cell,  an  epitome  of  all  the 
fundamental  life  processes  which  we  shall  later  have  occasion 
to  review  in  their  complex  expressions  in  the  higher  animals 
and  plants. 

Unicellular  green  plants  are  distributed  all  over  the  world 
and  adapted  to  a  great  variety  of  conditions.  We  find  them, 
for  example,  forming  green  coatings  on  the  bark  of  trees, 
scums  on  puddles  and  ponds,  or  being  blown  about  as  dust 
by  winds.  Of  the  many  hundreds  of  species  we  select 
Sphaerella  lacustris  because  it  can  readily  be  obtained  and 
kept  for  observation,  and  because  its  life  history  has  been 
carefully  studied. 

A.  STRUCTURE  AND  LIFE  HISTORY  OF  SPHAERELLA 

A  single  Sphaerella  is  invisible  or  barely  visible  to  the  naked 
eye,  but,  like  many  another  microscopic  form,  it  makes  up  in 
numbers  for  the  small  size  of  the  individual,  and  sometimes 
gives  a  stagnant  pool  of  rain-water  a  bright  green  or  red 
color.  Sphaerella  has  a  complicated  LIFE  CYCLE,  or  series  of 
forms  which  it  assumes  under  different  conditions,  chiefly 
environmental.  We  shall  take  as  the  initial  stage  for  descrip- 

30 


METABOLISM   OF   GREEN   PLANTS  31 

tion  the  so-called  PORMANT  FORM  which  may  be  assumed  when 
the  water  in  which  it  has  been  living  dries  up.  In  this 
condition  the  organism  consists  of  a  spherical  mass  of  pro- 
toplasm near  the  center  of  which  is  a  rather  large  nucleus. 
The  protoplasm,  which  appears  greenish  or  reddish  for  rea- 
sons to  be  discussed  later,  is  enclosed  within  a  distinct,  rigid 
cell  wall.  This  has  been  secreted  by  the  cell  and  is  com- 
posed of  CELLULOSE,  a  carbohydrate  which  is  especially 
characteristic  of  plant  cells.  It  is  evident  that  the  organism 
is  a  single  cell.  (Fig.  9.) 

Sphaerella  in  this  phase  is  able  to  withstand  unfavorable 
conditions  for  several  years  at  least.  All  the  life  processes 
of  the  protoplasm  are  reduced  to  the  lowest  ebb;  so  low  that 
it  is  difficult  to  demonstrate  any  chemical  change  whatsoever 
going  on.  Life  in  a  dormant  condition  is  not  peculiar  to 
Sphaerella,  but  is  quite  a  characteristic  phase  in  the  life  of 
many  animals  and  plants,  being  most  familiar  to  us  in  the  case 
of  plant  seeds,  some  of  which  are  known  to  retain  their  vital- 
ity for  nearly  a  century  under  proper  conditions. 

When  dormant  specimens  of  Sphaerella  are  placed  in  rain 
water  in  the  sunlight  active  life  shortly  is  resumed.  The  cell 
wall  swells  up  and  the  protoplasm  within  divides  twice,  with 
the  result  that  four  smaller  but  otherwise  essentially  similar 
cells,  known  as  SPORES,  take  the  place  of  the  original  cell  and 
are  set  free  by  the  rupture  of  its  wall.  The  four  daughter 
cells  soon  become  more  or  less  oval  in  outline  and  secrete 
cellulose  walls  of  their  own.  The  cell  walls  do  not  fit  closely 
about  the  body  of  protoplasm,  termed  the  PROTOPLAST,  but 
are  separated  from  it'  by  a  liquid-filled  space,  or  vacuole, 
except  where  cytoplasmic  strands  extend  through  the  vacuole 
to  the  wall.  But  a  more  remarkable  change  occurs  at  the 
same  time  —  two  long  slender  cytoplasmic  strands  are 
developed  from  the  more  pointed  end  of  the  cell,  and  these, 


32 


FOUNDATIONS   OF   BIOLOGY 


FIG.  9.  — Life  history  of  Sphaerella  lacustris.  a,  b,  c,  d,  asexual  cycle;  a,  w,x,  y,  z, 
sexual  cycle,  a,  dormant  cell  enclosed  within  a  protective  cyst  wall  which  has 
ruptured  to  allow  the  enclosed  protoplast  to  escape;  6,  division  of  the  protoplast  to 
form  four  spores  (c)  each  of  which  grows,  develops  two  flagella,  and  assumes  the  typical 
'adult'  free  living  form  of  Sphaerella  (d).  This  may  divide  many  times,  but  each  cell 
eventually  assumes  the  dormant  form  (a)  again.  Under  other  circumstances  the  proto- 
plast from  the  dormant  form  may  divide  until  32  or  64  small  cells  (w)  are  formed.  These 
make  their  escape  and  are  gametes  (x)  since  they  fuse  in  pairs  (j/).  The  composite  cell 
resulting  from  fertilization  is  a  zygote  (z)  which  soon  forms  a  cyst  wall  and  assumes  the 
dormant  phase. 


METABOLISM   OF   GREEN   PLANTS  33 

passing  through  the  cellulose  wall,  extend  for  some  distance 
into  the  surrounding  water.  These  threads  of  cytoplasm,  or 
FLAGBLLA,  lash  vigorously  and  pull  the  cell  rapidly  through 
the  water.  The  activity  of  the  flagella  is  one  expression  of  a 
fundamental  property  of  protoplasm,  CONTKACTILITY,  which 
is  exhibited  in  its  most  specialized  form  in  the  muscles  of 
the  higher  animals. 

Returning  now  to  the  life-history  of  Sphaerella.  The  four 
free-swimming  individuals,  which  have  arisen  from  the  parent 
dormant  phase,  may  each  divide  many  times  so  that  instead 
of  four  there  may  be,  before  long,  thousands  of  flagellated 
cells,  all  direct  lineal  descendants  of  the  original  resting  cell. 
If  this  number  seems  high,  one  only  has  to  determine  how 
many  cells  there  would  be  at,  say,  the  twenty-fifth  generation 
by  raising  2  to  the  25th  power.  Sooner  or  later,  however, 
these  active  cells  withdraw  their  flagella  and  again  become 
dormant  forms. 

But  Sphaerella  is  still  more  versatile.  Now  and.  then, 
probably  influenced  by  environmental  conditions,  the  proto- 
plasm within  the  wall  of  a  spherical  dormant  form  divides 
rapidly  into  32  or  64  relatively  small  cells  which,  when  set 
free,  are  termed  GAMETES.  These  differ  structurally  from 
the  active  form  already  described  chiefly  by  the  absence  of 
the  prominent  cell  wall  and  vacuole.  But  it  is  the  behavior 
rather  than  the  structure  of  these  small  cells  which  is  char- 
acteristic. After  swimming  about  for  a  time  by  means  of 
their  flagella,  they  come  together  in  pairs,  the  two  cells  of  a 
pair  completely  fusing  —  nucleus  with  nucleus  and  cyto- 
plasm with  cytoplasm  —  to  form  a  single  cell,  or  ZYGOTE, 
with  four  flagella.  Soon  the  individual  absorbs  its  flagella 
and,  secreting  about  itself  a  heavy  cell  wall,  enters  upon  a  dor- 
mant stage  with  the  characteristics  and  potentialities  de- 
scribed above*. 


34  FOUNDATIONS   OF   BIOLOGY 

It  is  clear  that  the  various  forms  which  follow  one  another 
arise  by  cell  division  in  every  case,  though  this  is  interrupted 
once  by  just  the  opposite  process  —  the  complete  cytoplasmic 
and  nuclear  fusion  of  two  distinct  cells  to  form  one  cell. 
This  is  the  process  of  FERTILIZATION,  an  expression  of  a  funda- 
mental phenomenon  of  protoplasm  at  the  basis  of  sex  and 
sexual  reproduction,  which  we  shall  consider  at  length  later. 

Such  is  the  history  of  Sphaerella.  It  is  apparent  that  the 
sequence  of  diverse  forms  which  arise  from  one  another 
constitute  a  life  cycle,  and  although  each  individual  cell  in 
the  cycle  is  a  Sphaerella,  nevertheless  the  plant  called 
Sphaerella  lacustris  comprises  all  the  forms  assumed.  From 
one  viewpoint  we  may  look  upon  the  cycle  as  forming  an 
individual  of  a  different  or  higher  order  —  an  individual  the 
component  cells  of  which  are  separate. 

B.  METABOLISM  IN  SPHAERELLA 

We  ,now  turn  our  attention  from  the  structure  and  life 
history  of  Sphaerella  to  the  point  it  was  chosen  especially  to 
illustrate  —  the  metabolism  of  green  plants.  It  may  appear 
to  the  reader  that  a  tree  or  shrub  might  with  more  propriety 
be  taken  as  the  example  of  a  typical  plant,  but,  since  the 
fundamental  distinction  between  animals  and  plants  is 
chiefly  a  question  of  -metabolism,  there  are  advantages  in 
studying  it  in  a  single  cell,  where  one's  attention  is  not  dis- 
tracted by  root,  stem,  and  leaf. 

Since  Sphaerella  lives,  grows,  and  multiplies  in  pools  of 
water  exposed  to  sunlight,  it  is  to  this  environment  that  we 
must  look  for  the  materials  which  it  turns  into  protoplasm, 
and  the  energy  by  which  it  makes  the  transformation.  And 
further,  since  the  organism  is  enclosed  in  a  cell  wall,  its  income 
and  outgo  of  materials  must  be  in  solution  in  ordei  to  pass 
through. 


METABOLISM   OF   GREEN   PLANTS  35 

1 .   Food  Making 

In  short,  Sphaerella  takes  materials  from  its  surroundings 
in  the  form  of  simple  compounds,  as  carbon  dioxide, 
water,  and  mineral  salts,  which  are  relatively  stable  and  there- 
fore practically  devoid  of  energy,  and,  through  the  radiant 
energy  of  sunlight,  shifts  and  recombines  their  elements  in  such 
a  way  that  products  rich  in  potential  energy  result.  Sphae- 
rella thus  exhibits  the  prime  diagnostic  characteristic  of  green 
plants  —  the  power  to  construct- its  own  foodstuffs. 

The  key  Fo  this  power  of  chemical  synthesis  by  light  — 
PHOTOSYNTHESIS  —  resides  in  a  complex  chemical  substance 
called  CHLOROPHYLL.1  This  pigment,  which  is  segregated  in 
special  cytoplasmic  bodies  known  as  CHLOROPLASTIDS,  gives 
to  Sphaerella  during  its  active  phases  and  to  the  foliage 
of  plants  in  general  their  characteristic  green  color.  The 
chlorophyll  arrests  and  transforms  a  small  part  of  the  energy 
of  the  sunlight,  which  impinges  upon  it,  in  such  a  way  that 
the  protoplasm  can  employ  this  energy  for  food  synthesis. 

The  first  great  step  in  the  constructive  process  is  a  com- 
bination of  carbon  with  hydrogen  and  oxygen  to  form  a 
carbohydrate.  Sphaerella  gets  these  elements  from  carbon 
dioxide  and  water  by  a  process  of  molecular  disruption.  We 
know  that  when  charcoal,  for  instance,  is  burned,  carbon  and 
oxygen  unite  to  form  carbon  dioxide,  and  energy  in  the  form 
of  light  and  heat  is  liberated.  Obviously  Sphaerella  must 
employ  an  equal  amount  of  energy  in  separating  the  carbon 
and  oxygen  of  carbon  dioxide;  that  is,  in  overcoming  their 
chemical  affinity.  And  this  kinetic  energy  which  the  plant 
employs  is  then  represented  in  the  chemical  potential  which 
exists  between  the  oxidizable  carbon  and  free  oxygen  —  it  is 

1  A  rough  approximation  of  the  formula  of  chlorophyll  has  been  given  as:  (MgN4- 
Cx-HaoOO)  (COOCHs)  (COOC2oH39).  A  slight  chemical  modification  of  chlorophyll 
results  in  hematochrome,  which  gives  at  certain  times  the  reddish  tinge  to  Sphaerella. 


36  FOUNDATIONS   OF   BIOLOGY 

potential  energy.     Thus  the  plant  in  sunlight  is  continually 
separating  the  carbon  from  the  oxygen  of  carbon  dioxide. 
The  oxygen  is  liberated  as  free  oxygen  while  the   carbon 
which  has  been  separated  from  the  oxygen  is  combined  with 
molecules  of  water  to  form  carbohydrates  —  grape  sugar 
(glucose)  and  fruit  sugar  (fructose). 
The  conventional  equation  for  this  reaction  is: 
6  C02      +     6H2O  C6H12O6      +      6O2 

(carbon  dioxide)  (water)         (glucose  or  fructose)         (free  oxygen) 

It  should  be  emphasized,  however,  that  the  processes  in- 
volved are  by  no  means  so  simple  as  is  implied  above;  but 
since  there  is  little  conclusive  data  in  regard  to  the  details, 
the  equation  as  stated  affords  a  formal  explanation  which  is 
adequate  for  the  present  discussion. 

The  first  great  step  in  food  synthesis,  the  formation  of  a 
sugar,  having  been  accomplished,  the  green  plant  trans- 
forms the  sugar  and  stores  it  as  starch  for  future  use  as  fuel 
or  ES  the  basis  of  further  synthesis.  Starch  is  the  first  visible 
product  of  photosynthesis. 

We  have  seen  that  the  characteristic  of  proteins  as  com- 
pared with  carbohydrates  (sugars,  starches)  is  the  presence 
of  nitrogen,  and  this  element  must  be  added  to  the  CHO 
basis  already  constructed  as  the  next  step  toward  protein 
synthesis.  The  green  plant  not  only  can,  but  must  employ 
nitrogen  in  such  simple  combinations  as  nitrates,  and  this  is 
a  fact  of  prime  importance,  for  typically,  as  will  appeal- 
later,  animals  and  most  colorless  plants  require  nitrogen 
in  more  complex  combinations.  By  the  addition,  then,  of 
nitrogen  to  the  carbohydrate  basis  a  very  simple  nitrogen 
compound,  such  as  an  amine  (e.g.,  asparagine  =  C^s^Oa),  is 
built  up,  which  may  be  transformed  into  a  protein  by  the  ad- 
dition of  sulfur  and  other  elements  secured  from  sulfates, 
phosphates,  etc.  Again,  we  do  not  know  how  this  is  done. 


METABOLISM   OF   GREEN   PLANTS  37 

or,  after  it  is  done,  how  the  protein  becomes  an  intrinsic  part 
of  the  living  material  itself.  So  we  attribute  it  to  synthesiz- 
ing ENZYMES.  These  are  chemical  bodies  which  are  only 
known  as  products  of  living  protoplasm  and  are  the  activat- 
ing agents  (catalytic  agents)  for  chemical  transformations 
in  which,  however,  they  themselves  take  no  integral  part. 
The  chemical  composition  and  constitution  of  enzymes  is 
undetermined. 

Sphaerella  thus  takes  the  raw  elements,  so  to  speak,  of 
living  matter  and  by  the  radiant  energy  of  sunlight,  which 
its  chlorophyll  traps,  constructs  carbohydrate,  protein,  pro- 
toplasm. In  other  words,  the  green  plant  is  a  synthesizing 
agent,  building  up  highly  complex  and  unstable  molecular 
aggregates  brimming  over  with  the  energy  received  from 

the  Sun. 

2.   Respiration 

So  the  green  plant,  whether  Sphaerella  or  Elm,  manu- 
factures its  own  food  and  itself!  But,  as  we  have  said  before, 
protoplasm  is  always  at  work  —  to  live  is  to  work  —  and 
this  means  expenditure  of  energy,  the  same  energy  which 
chlorophyll  has  secured  for  the  plant  and  stored  away  in  its 
food.  In  other  words,  the  food  must  be  oxidized  in  order  to 
release  the  energy,  and  for  this  the  plant  must  have  available 
a  supply  of  free  oxygen.  Sphaerella  obtains  this  oxygen  dis- 
solved in  the  water  and,  incidentally,  in  sunlight,  from  that 
liberated  through  photosynthesis.  The  process  involved, 
for  the  sake  of  simplicity,  may  be  represented  by  the  equa- 
tion: 

C6H12O6  +  6  O2  =  6  CO2  +  6  H2O 

which,  it  will  be  noted,  is  the  reverse  of  the  equation  for 
photosynthesis.  This  intake  of  free  oxygen  by  the  cell  and 
outgo  of  carbon  dioxide  and  water,  the  chief  products  of 
combustion,  is  known  as  RESPIRATION.  It  is  an  interchange 


38  FOUNDATIONS   OF   BIOLOGY 

of  gases  between  the  living  matter  and  its  surroundings  which 
is  not  only  characteristic  of  Sphaerella  and  all  green  plants, 
but  of  all  living  things.  Plants  breathe  just  as  truly  as 
animals,  though  the  active  life  of  most  of  the  latter  requires 
a  more  or  less  elaborate  respiratory  apparatus  in  order 
that  an  adequate  gaseous  interchange  may  be  effected  with 
the  necessary  rapidity. 

Thus  the  green  plant  may  be  regarded  as  a  chemical 
machine  for  the  transformation  of  energy  —  the  radiant 
energy  from  the  Sun  —  into  life  work;  the  matter  and  energy 
which  enters,  forms,  and  leaves  the  organism  obeying,  to  the 
best  of  our  knowledge,  the  fundamental  laws  of  matter  and 
energy  of  the  non-living  world. 

We  have  now  obtained  some  idea  of  one  living  organism, 
Sphaerella  lacustris,  a  green  plant  reduced  to  the  simplest 
terms  —  a  single  cell  provided  with  chlorophyll.  And  we 
have  seen  that  this  chlorophyll  is  the  key  to  the  photo- 
synthetic  activity  of  the  green  plant.  In  other  words,  the 
expression  'green  plant'  does  not  refer  specifically  to  the 
color  of  a  plant  (in  some  cases  it  may  appear  red,  as  in 
Sphaerella  under  certain  conditions),  but  to  the  fact  that 
there  is  present  a  complex  pigment  functionally  similar  to 
chlorophyll  by  virtue  of  which  the  plant  is  a  constructive 
agent  in  nature.  It  has  the  power  to  manufacture  its  own 
foodstuffs  from  relatively  simple  compounds  largely  devoid 
of  energy  and,  in  particular,  is  able  to  utilize  nitrogen  in  the 
form  of  nitrates. 

We  pass  now  from  the  essentially  constructive  agents  in 
nature  to  the  chiefly  destructive;  from  the  collectors  of 
energy  to  the  energy  dissipators;  from  the  green  plants  to 
animals  and  to  'colorless'  plants. 


CHAPTER  V 
METABOLISM  OF  ANIMALS 

The  most  important  discoveries  of  the  laws,  methods,  and 
progress  of  Nature  have  nearly  always  sprung  from  the  exami- 
nation of  the  smallest  objects  which  she  contains,  and  from 
apparently  the  most  insignificant  enquiries.  —  Lamarck. 

THERE  is  probably  no  better  introduction  to  the  study 
of  the  biology  of  an  animal  than  that  afforded  by  PARA- 
MECIUM,  a  common  organism  of  ponds,  ditches,  and  decaying 
vegetable  infusions.  Paramecium  is  a  representative  of 
some  10,000  kinds  of  single-celled  animals,  or  PROTOZOA. 
Members  of  this  group  are  found  in  almost  every  niche  in 
nature  and,  like  the  PROTOPHYTA,  as  the  unicellular  plants 
are  sometimes  called,  are  important  because  in  numbers  there 
is  strength. 

A.  STRUCTURE  AND  LIFE  HISTORY  OF  PARAMECIUM 

Paramecium  is  a  giant  among  the  Protozoa,  though  just 
visible  to  the  naked  eye  as  a  whitish  speck  if  the  water  in 
which  it  is  swimming  is  properly  illuminated.  But  to  make 
out  the  details  of  structure  it  is  necessary  to  magnify  it 
several  hundred  times.  This  done,  it  appears  as  a  more  or 
less  cigar-shaped  organism  which  one  would  not  consider, 
at  first  glance,  a  single  cell  because  it  shows  highly  differen- 
tiated parts.  However,  careful  study  reveals  the  fact  that  the 
organism  really  consists  of  a  single  protoplasmic  unit  differ- 
entiated into  cytoplasm  and  nucleus,  though  each  of  these 
regions  shows  specializations.  The  nuclear  material,  instead 

30 


40 


FOUNDATIONS    OF   BIOLOGY 


of  forming  a  single  body  as  it  does  in  most  cells,  in  Parame- 
cium  is  distributed  in  two  parts:  a  larger  body,  or  MACRO^T 
NUCLEUS,  and  a  smaller  body,  or  MiCRONUCLEUs.1  Strictly\ 

speaking,  the  macronucleus  and  \ 
micronucleus  together  constitute    \ 
the  nucleus  of  the  cell,  and  rep- 
resent    a    sort    of    physiological, 
division  of  labor  in  the  chromatin  y\ 
complex.     But  it  is  in  the  cyto-, 
plasm  that  specialization  is  most 
conspicuous.     Not  only  are  there 
general  differentiations  into  ecto- 
plasm and  endoplasm,  but  these 
regions   also    have   local  speciali- 
zations  such   as   CILIA   for    loco- 
motion, TRICHOCYSTS  for  defense, 

PERISTOME,     MOUTH,    and    GULLET 

for    the    intake     of    solid    food, 
GASTRIC  VACUOLES  for  digestion, 
and   CONTRACTILE   VACUOLES   for  / 
excretion.     And  withal,  recent  in-/ 
vestigations  indicate  that  various' 
parts  of  the  cell  are  coordinated 


FIG.  10.  —  Paramecium  calkinsi. 
Diagrammatic,  a,  contractile  vac- 
uole  surrounded  by  radiating 
canals;  b,  macronucleus;  c,  mouth; 
d,  undulating  membrane  extending 
lengthwise  in  gullet;  e,  gastric 
vacuole  in  process  of  formation,  at 
end  of  gullet;  /,  contractile  vacuole, 
fully  formed;  g,  gastric  vacuoles; 
h,  endoplasm;  i,  micronuclei;  j, 
peristome  and  peristomial  cilia;  k, 
trichocysts  in  ectoplasm;  /,  cilia. 


apparatus. 


by    a    *  neuromotor ' 

(Fig.  10.) 
Paramecium,    under   favorable 

conditions,  grows  rapidly  and, 
when  it  has  attained  the  size  limit  characteristic  of  the 
species,  cell  division  takes  place,  with  the  result  that  from 
the  single  large  cell  there  are  formed  two  smaller  individuals 
which  soon  become  complete  in  all  respects.  These,  in  turn, 


1  The  several  species  of  Paramecium  differ  in  regard  to  micronuclear  number;  e.g., 
P.  caudatum  has  one  micronucleus,  and  P.  aurelia  and  P.  calkinsi  have  two  micronuclei. 


METABOLISM    OF   ANIMALS 


41 


grow  and  repeat  the  process  in  about  ten  hours  so  that,  as 
in  the  case  of  Sphaerella,  within  a  few 
days  the  original  Paramecium  has  divided 
its  individuality,  so  to  speak,  among   a 
multitude  of  descendants.     (Fig.  11.) 

This  process  of  multiplying  by  dividing 
can  go  on  indefinitely  under  optimum  en- 
vironmental conditions.  But  periodically 
Paramecium  undergoes  an  internal  nuclear 
reorganization  process  (ENDOMIXIS).  Also 
now  and  then  individuals  temporarily  fuse 
in  pairs  and  interchange  nuclear  material 
(CONJUGATION)  —  an  expression  of  the 
same  fundamental  sex  phenomenon  which 
is  exhibited  in  Sphaerella.  (Figs.  12,  130, 
131.) 


B.    METABOLISM  IN  PARAMECIUM 


1 1 .  —  Parame- 


cium aurelia,  dividing. 
N,  N',  macronucleus ; 
n,  n',  the  two  dividing 
micronuclei ;  o,  o', 
mouth.  (After  Hertwig.) 


Paramecium  thus  affords  some  idea  of  the  complexities  of 
structure  and  function  which  a  cell  may  exhibit  when  it  forms 
the  whole  animal  organism.  The  Pro- 
tozoa are  the  simplest,  though  by  no 
means  simple,  animals.  But  the  great 
structural  differences  between  Sphaerella 
and  Paramecium,  though  to  a  certain 
extent  representative  of  plants  on  the 
one  hand  and  animals  on  the  other,  are 
not  essentially  diagnostic,  because,  as  we 
have  suggested  before,  in  the  last  analysis 
FIG.  12  —Position  as-  ^  *s  a  matter  of  metabolism.  And  it  is 


by  conjugating   largely  for  this  reason  that  Sphaerella  and 

Paramecia. 

Paramecium,  organisms  shorn  of  all,  or 
nearly  all,  non-essentials,  have  been  selected  as  illustrations. 


42  FOUNDATIONS    OF   BIOLOGY 

1.  Food  Taking 

The  food  of  Paramecium  is  chiefly  microscopic,  colorless 
plants  known  as  BACTERIA  which  are  present  in  countless 
numbers  in  decaying  vegetable  infusions.  As  Paramecium 
swims  about  by  means  of  its  cilia,  a  current  of  water  laden 
with  Bacteria  is  whirled  down  the  peristome  on  one  side  of 
the  animal  and  some  passes  through  the  mouth  and  gullet 
into  the  endoplasm.  Here  the  Bacteria,  surrounded  by  a 
droplet  of  water,  form  a  gastric  vacuole,  into  which  the  endo- 
plasm secretes  chemical  substances  (enzymes,  etc.)  which 
gradually  break  down  —  that  is,  digest  —  the  complex 
proteins,  carbohydrates,  etc.,  of  the  plant  cells.  Finally, 
this  material  which  shortly  before  was  Bacteria  protoplasm 
is  incorporated  into  Paramecium  protoplasm  —  matter  and 
energy  is  supplied  and  the  animal  lives  and  grows. 

This  is,  in  most  regards,  a  strikingly  different  condition 
from  that  which  we  have  seen  in  Sphaerella.  In  Paramecium 
solid  particles  of  food  —  Bacteria  —  are  taken  into  the  cell, 
and  since  the  chief  organic  constituents  of  protoplasm  are^ 
proteins,  associated  with  carbohydrates  and  fats,  it  is  clear 
that  the  income  of  the  animal  organism  is,  unlike  that  of  the 
green  plant,  chiefly  ready-made  complex  foodstuffs.  In 
other  words,  Paramecium,  like  all  animals,  requires 
relatively  complex  chemical  compounds  rich  in  potential 
energy:  proteins,  carbohydrates,  and  fats.  Of  these,  pro- 
teins or  their  constituent  amino  acids  are  absolutely  indis- 
pensable because  it  is  only  from  this  source  that  nitrogen 
is  available  for  the  animal.  But  the  green  plant,  through 
its  chlorophyll  apparatus,  is  able  to  take  materials  largely 
devoid  of  energy  and  to  rearrange  them  and  endow  them' 
with  potential  energy  which  it  has  received  in  the  kinetic 
form  from  sunlight. 


METABOLISM   OF  ANIMALS  43 

2.   Respiration  and  Excretion 

Of  course,  during  life,  the  animal,  like  the  green  plant,  is 
continually  breaking  down  its  food  and  its  own  protoplasm 
by  a  process  of  combustion  which  involves  an  intake  of  free 
oxygen  and  the  liberation  of  carbon  dioxide  and  water. 
Nitrogenous  wastes,  chiefly  UREA,  as  well  as  inorganic  salts, 
are  also  excreted.  So  the  animal,  like  the  plant,  returns  to 
its  environment  the  elements  in  simple  combinations  which 
are  devoid  or  nearly  devoid  of  energy.  We  have  stated  that 
green  plants  are  essentially  constructive  and  animals  es- 
sentially destructive  agents  in  nature.  It  is  now  apparent 
that  green  plants  are  both  constructive  and  destructive,  while 
animals  are  essentially  destructive. 

A  little  consideration  of  the  income  and  outgo  of  green 
plants  and  animals  will  show  that,  although  the  animals  are 
dependent  on  the  plants  for  their  complex  foodstuffs,  they  do 
not  return,  for  example,  the  nitrogen  to  the  outer  world  in 
a  form  simple  enough  to  be  available  for  green  plants.  In 
other  words  the  urea,  (NH2)2CO,  which  still  has  a  little 
energy  left  which  the  animal  is  unable  to  extract,  must  be 
transformed  into  nitrates. 

Furthermore,  since  plants  die,  which  are  not  consumed  by 
animals,  and  animals  die,  which  are  not  devoured  by  other 
animals,  large  stores  of  matter  and  energy  are  locked  up  in  the 
complex  compounds  of  their  dead  tissues.  Clearly,  there  must 
be  some  way  of  completing  the  cycle  of  the  elements,  for  if 
there  were  not,  life,  as  we  know  it,  could  not  have  continued 
long  on  the  Earth.  This  gap  is  filled  by  the  so-called  COLOJI-^ 
LESS  PLANTS,  that  is  plants  which,  because  chlorophyll  is  not 
present,  lack  the  power  of  photosynthesis  and  so  in  most 
cases  are  dependent  for  food  on  more  complex  substances 
than  green  plants  demand,  though  not  so  complex  as  animals 
require. 


CHAPTER  VI 
METABOLISM  OF  COLORLESS  PLANTS 

Nature,  which  governs  the  whole,  will  soon  change  all  things 
which  thou  seest,  and  out  of  their  substance  will  make  other 
things  and  again  other  things  from  the  substance  of  them,  in 
order  that  the  world  may  be  ever  new.  —  Marcus  Aurelius. 

As  representative  of  the  diverse  types  of  colorless  plants 
which,  lacking  chlorophyll  or  a  functionally  similar  pigment, 
are  without  the  power  of  photosynthesis,  we  select  the  vast 
group  known  as  the  BACTERIA.  For  reasons  that  will  appeal- 
later,  it  is  not  practical  to  focus  attention  on  one  particular 
species  of  Bacteria,  as  we  have  just  done  in  considering  green 
plants  and  animals.  Instead  we  shall  discuss  in  very  general 
terms  the  group  as  a  whole,  referring  now  and  then  to  special 
kinds  of  Bacteria  to  illustrate  particular  points. 

A.    THE  BACTERIA 

The  wide  distribution  of  the  Protozoa  is  exceeded  by  the 
Bacteria.  Representatives  are  literally  found  everywhere: 
floating  with  dust  particles  in  the  air;  in  salt  and  fresh  water: 
in  the  water  of  hot  springs;  frozen  in  ice;  in  the  upper  layers 
of  the  soil;  and  in  the  bodies  of  plants  and  animals.  Bacteria 
have  received  a  considerable  notoriety  under  the  names  of 
' microbes '  and  'germs,'  owing  to  the  fact  that  certain  types 
get  a  living  within  the  human  body  as  parasites  and  bring 
about  disturbances,  chiefly  chemical,  which  we  interpret  as 
disease.  But  aside  from  these  forms,  which  are  relatively 
few  in  number,  human  life  and  life  in  general  on  the  Earth 

44 


METABOLISM  OF  COLORLESS  PLANTS 


45 


could  not  long  continue  without  their  services.     It  is  this 
aspect  of  the  Bacteria  which  concerns  us  at  present. 

Among  the  Bacteria  are  the  smallest  organisms  known. 
Some  species  are  less  than  one  fifty-thousandth  of  an  inch 
in  length  and  much  less  in  breadth.  None  of  the  typical 
forms  comes  within  the  range  of  unaided  vision,  —  indeed 
there  is  room  and  to  spare  for  thousands  of  millions  of  Bac- 
teria to  live  in  a  thimble-full  of  sour  milk.  The  small 
size  and  similarity  of  structure  of  many  of  the  Bacteria 
render  their  study  particularly  difficult,  and  accordingly 


0® 


CD 


FIG.  13. — Chief  types  of  Bacteria.  A,  cocci;  B,  bacilli;    C,  spirilla;  D,  branched 
filamentous  form.     (From  Buchanan.) 

they  are  grouped  and  classified  largely  on  the  basis  of  chem- 
ical changes  which  they  produce,  rather  than  on  structural 
characteristics.  However,  there  are  three  chief  morphologi- 
cal types:  the  rod -like  forms  or  BACILLI;  the  spherical  forms 
or  cocci;  and  the  spiral  forms  or  SPIRILLA.  Bacilli  or  cocci 
may  be  associated  in  linear,  branching,  or  plate-like  series, 
or  grouped  together  in  colonies.  (Fig.  13.) 

The  individual  bacterium  is  generally  regarded  as  a  single 
cell  though  in  most  species  there  is  no  definite  nuclear  body; 
the  chromatin  material  being  distributed  in  the  form  of 
granules  throughout  the  cytoplasm.  A  cell  wall  chemically 
similar  to  protein  is  usually  present.  Some  forms  show 
active  movements  by  means  of  prolongations  of  the  cytoplasm, 


46 


FOUNDATIONS    OF   BIOLOGY 


or  flagella,  as  in  the  case  of  the  common  Spirillum  of  decay- 
ing vegetable  infusions.    (Fig.  14.) 

Reproduction  is  by  a  process  of  cell  division  which,  under 
favorable  conditions,  may  occur  as  often  as  every  half  hour. 
The  vast  multitude  of  cells  thus  produced  before  long  exhaust 
the  food  supply  and  contaminate  with  excretion  products 
the  medium  in  which  they  are  living,  so  that  further  growth  is 
inhibited.  Under  these  circumstances  the  protoplasm  within 
the  cell  wall  ordinarily  assumes  a  spherical  form  and  secretes 


FIG.  14.  — Types  of  flagellation  in  Bacteria.     1,  without  flagella  (at richou.s  forms); 
2,  3,  4,  5,  with  flagella  (trichous  forms).      (From  Buchanan.) 

a  protecting  coat  about  itself,  and  thus  enters  upon  a  resting 
state.  In  this  spore  form  the  Bacteria  can  withstand  drying 
and  variations  in  temperature  to  which  in  the  active  state 
they  would  readily  succumb,  and  thereby  the  organisms  tide 
over  periods  of  unfavorable  conditions  and  are  ready  to  start 
active  life  again  when  the  opportunity  presents  itself.  (Fig. 
160.) 

B.     CYCLE  OF  THE  ELEMENTS  IN  NATURE 
We  have  seen  that  carbon  dioxide  is  the  source  from  which 
green  plants  derive  the  carbon  which  they  synthesize  into 
carbohydrates,  fats,  and  proteins.     Animals  directly  or  in- 


METABOLISM  OF  COLORLESS  PLANTS     47 

directly  feed  on  plants  so  that  the  ultimate  source  of  the 
carbon  of  animals  is  likewise  the  carbon  dioxide  of  the  atmos- 
phere. Although  both  plants  and  animals  by  their  respir- 
atory process  are  continually  returning  to  the  outer  world 
some  of  this  carbon  as  carbon  dioxide,  it  is  evident  that 
relatively  enormous  amounts  of  carbon  are  nevertheless  being 
taken  out  of  circulation  and  locked  up  in  the  bodies  of  the 
plants  and  animals.  For  example,  it  has  been  estimated 
that  about  one  half  the  weight  of  a  dried  tree  trunk  is  con- 
tributed by  carbon. 

The  same  general  segregation  is  going  on  in  regard  to 
nitrogen.  The  green  plants  take  it  in  the  form  of  nitrates, 
for  instance,  and  store  it  away  in  the  proteins;  and  again 
animals  get  their  nitrogen  from  plant  proteins,  so  that  the 
ultimate  source  of  the  animal  nitrogen  is  the  same.  In  a 
somewhat  similar  manner  we  might  trace  the  fate  of  the 
other  chemical  elements  necessary  for  protoplasm,  but  that 
of  carbon  and  nitrogen  is  particularly  striking  and  instructive, 
and  is  sufficient  to  illustrate  the  fact  that  although  both 
green  plants  and  animals  are  continually  taking  elements 
from  and  returning  them  to  their  environment,  nevertheless 
more  is  taken  away  than  is  returned.  (Figs.  15,  16.) 

The  agents  which  restore  to  the  inorganic  world  the  ele- 
ments removed  by  green  plants  and  animals  are  the  colorless 
plants,  chief  among  which  are  the  Bacteria.  As  we  know, 
when  an  animal  or  plant  dies,  decay  sets  in  almost  immedi- 
ately; that  is,  the  complex  chemical  compounds  are  slowly 
but  surely  reduced  to  simpler  and  simpler  forms  until  '  dust ' 
remains.  Although  undoubtedly  many  of  these  compounds 
would  automatically,  so  to  speak,  tend  to  simplify,  never- 
theless this  is  not  only  hastened,  but  chiefly  carried  out  by 
organisms  of  decay  such  as  the  Bacteria.  Through  enzymes, 
or  ferments,  which  they  form,  FERMENTATION  occurs.  The 


48 


FOUNDATIONS   OF   BIOLOGY 


carbohydrates  and  fats  are  resolved  into  carbon  dioxide  and 
water,  and  the  proteins  are  reduced  to  carbon  dioxide,  water, 
and  ammonia  (NH3)  or  free  nitrogen,  while  the  nitrogenous 
waste  (urea,  etc.)  of  animals  is  similarly  broken  down.  Prac- 
tically all  of  these  long  series  of  chemical  reactions  are  carried 
on  by  different  kinds  of  Bacteria.  Most  green  plants,  how- 
ever, take  their  nitrogen  chiefly  in  the  form  of  nitrates  and 


•  Dead 
Organisms 

Living 
Animals 


Bacterial 
Decay 


Carbohydrates, 
Proteins,  Fats, 
in  Green  Plants 


\ 


Fermentation 
and  Animal 
Respiration 


Intermediate 
Decomposition 
Products 


FiG.  15.  — The  Carbon  Cycle.     A  schematic  representation  of  the  circulation 
of  carbon  in  nature. 

accordingly  we  find  that  another  type  of  Bacteria  (NITRITE 
BACTERIA)  acts  upon  the  ammonia  and  transforms  it  into 
nitrous  acid  (HNO2).  After  certain  chemical  reactions  in 
the  soil,  forming,  e.g.,  potassium  nitrite  or  ammonium  nitrite, 
still  another  type  (NITRATE  BACTERIA)  oxidizes  the  nitrites 
into  nitrates  (e.g.,  KNO3  or  NH^Oa),  so  that  again  this 
nitrogen  is  in  a  form  which  is  available  for  green  plants. 

But,  still  confining  our  attention  to  the  nitrogen,   it  is 
obvious  that  there  is  a  leak  from  this  cycle,  since  some  of  the 


METABOLISM  OF  COLORLESS  PLANTS 


49 


nitrogen  in  the  form  of  ammonia  or  free  nitrogen  escapes 
to  the  atmosphere.  The  greatest  loss,  however,  is  brought 
about  by  a  group  of  DENITRIFYING  BACTERIA  whose  activities 
are  largely  spent  in  changing  nitrates  into  gaseous  nitrogen 
which  escapes  into  the  air,  and  so  putting  it  beyond  the  reach 
of  green  plants  and  animals.  Fortunately  there  is  also  a 
special  group  of  NITROGEN-FIXING  BACTERIA  which  rescue 


.Animal, 
Proteins 


Proteins 
of  Green  Plants 


itrite     Ammonia 
itrites^±£i^7 

Denitrifying 
Bacteria 


FIG.  16.  —  The  Nitrogen  Cycle.     A  schematic  representation  of  the  circula- 
tion of  nitrogen  in  nature. 

the  nitrogen  from  the  atmosphere  and  return  it  to  the  cycle 
of  elements  in  living  nature.  These  organisms  inhabit 
the  soil  or  little  nodules  which  they  produce  on  the 
rootlets  of  leguminous  plants,  such  as  beans,  clover,  and 
alfalfa;  and  this  accounts  for  the  fact,  long  known  but  not 
understood,  that  these  plants  when  plowed  under  are  par- 
ticularly efficient  in  enriching  the  soil.  In  brief,  there  is  a 
cycle  of  the  elements  in  nature  through  green  plants  and 
animals  and  back  again  to  the  inorganic  world  through  the 


50  FOUNDATIONS    OF    BIOLOGY 

Bacteria  and  other  colorless  plants.     Such  is  the  reciprocal 
nature  of  the  nutritive  processes  of  living  organisms. 

It  is  hardly  necessary  to  state  that  the  chemical  changes 
produced  by  the  Bacteria  are  either  the  direct  results  of,  or 
are  incidental  to,  the  process  of  nutrition  in  these  organisms. 
Therefore  the  material  taken  as  food  by  certain  groups  is 
relatively  complex,  for  example,  by  those  which  bring  about 
the  early  putrefactive  changes  in  proteins;  while  that  em- 
ployed by  others  is  very  simple  since  they  find  adequate 
chemical  combinations  less  complex  than  those  needed  by 
green  plants.  Indeed,  it  is  now  known  that  one  group  of 
Bacteria  is  able  to  utilize  carbon  dioxide  and  water  just  as 
do  green  plants.  But  instead  of  obtaining  energy  for  the 
synthesis  from  sunlight,  these  Bacteria  derive  it  from  chemi- 
cal energy  liberated  by  the  oxidation  of  substances  in  their 
environment.  This  process  is  referred  to  as  CHEMOSYN- 
THESIS,  in  contrast  with  photosynthesis,  and  although  it  is 
apparently  restricted  to  a  relatively  small  group  of  organ- 
isms, may  well  represent  the  most  primitive  method  of 
nutrition  from  which  all  the  others  have  been  derived  in  the 
evolution  of  life. 

C.    THE  HAY  INFUSION  MICROCOSM 

The  importance  of  the  complex  nutritional  interdependence 
of  organisms  in  general  and  the  cosmical  function  of  green 
plants  —  the  link  they  supply  in  the  circulation  of  the  ele- 
ments in  nature  —  may  be  emphasized  and  summarized  by 
a  brief  description  of  a  'hay  infusion.1 

Probably  nowhere  is  the  'web  of  life'  more  conveniently 
or  convincingly  exhibited  than  in  the  kaleidoscopic  sequence 
of  events  —  physical,  chemical,  and  biological  —  which  are 
initiated  when  a  few  wisps  of  hay  are  added  to  a  beaker  of 
water.  Apparently  the  chief  components  of  a  hay  infusion 


METABOLISM   OF   COLORLESS   PLANTS  51 

are  hay  and  water,  but  these  merely  supply  the  matter  and 
energy  for  the  interplay  of  various  forms  of  life.  Most  of 
these  are  beyond  the  scope  of  unaided  vision  though  chiefly 
responsible  for  the  obvious  changes  which  occur  from  day  to 
day  in  their  environment. 

Ordinary  tapwater,  for  instance,  contains  free  oxygen  and 
various  inorganic  salts  in  solution,  and  not  infrequently 
different  species  of  Bacteria,  unicellular  green  plants,  and 
Protozoa.  The  hay  soaking  in  the  water  contributes  soluble 
salts,  carbohydrates,  proteins,  etc.  It  also  supplies  many 
microscopic  animals  and  plants  which  have  adhered  to  it  in 
dormant  form  and  are  only  awaiting  suitable  surroundings 
to  assume  active  life  again. 

A  microscopical  examination  of  an  infusion  when  it  is  first 
made  shows  very  few  active  organisms,  but  within  a  day  or 
so,  depending  largely  on  the  temperature,  it  reveals  countless 
numbers  of  Bacteria  which  have  arisen  by  division  from  the 
relatively  small  number  of  dormant  and  active  specimens 
originally  present.  At  first  the  Bacteria  are  fairly  evenly 
distributed  in  the  infusion,  but  as  conditions  change,  largely 
through  the  chemical  and  physical  transformations  which 
they  themselves  bring  about,  those  species  which  can  employ 
oxygen  in  combined  form  (that  is,  in  chemical  compounds) 
find  existence  possible  and  competition  less  keen  at  the  bot- 
tom of  the  beaker,  while  those  types  of  Bacteria  which  are  de- 
pendent upon  free  oxygen  gather  nearer  the  surface  where  the 
supply  is  being  replenished  constantly  from  the  atmosphere. 

Up  to  this  point  the  life  of  our  microcosm  is  largely  bac- 
terial —  unicellular  SAPROPHYTIC  plants  which  employ  as 
food  the  complex  decomposition  products  of  the  proteins, 
etc.,  of  the  hay.  The  process  is  essentially  destructive  and 
the  simplified  products  are  represented  in  the  relatively 
simple  excretions  of  the  Bacteria. 


52  FOUNDATIONS    OF   BIOLOGY 

But  during  bacterial  ascendency  another  factor  has  been 
gradually  intruding  itself  almost  imperceptibly  into  the 
drama.  This  is  the  microscopic  animal  life  which  has 
been  multiplying  with  increasing  rapidity  as  conditions 
became  more  favorable,  and  forthwith  assumes  the  dominant 
life  phase  in  the  infusion.  Among  the  animal  forms,  the 
first  to  appear  are  exceedingly  minute  flagellated  Protozoa, 
known  as  Monads,  many  species  of  which  absorb  products 
of  organic  disintegration  brought  about  by  the  Bacteria, 
while  others  ingest  solid  food  —  the  Bacteria  themselves. 
Then  tiny  ciliated  animals,  close  relatives  of  Paramecium, 
appear  in  untold  numbers  and  feed  upon  the  Bacteria.  The 
dominance  of  these  smaller  ciliates  is  brought  to  an  end 
after  a  few  days  by  the  ascendency  of  larger  ciliates,  which, 
though  feeding  to  a  certain  extent  upon  the  already  greatly 
depleted  bacterial  population,  obtain  most  of  their  food  by 
eating  the  smaller  ciliates.  And  so  the  cycle  of  life  continues 
—  saprophytic  forms  gradually  being  replaced  in  dominance 
by  herbivorous  and  these  in  turn  by  carnivorous  organisms. 

But  obviously  this  chain  of  events  must  sooner  or  later 
come  to  an  end  through  the  dissipation  of  energy  brought 
about  by  the  metabolic  processes  of  the  colorless  plants  and 
animals.  Sooner  or  later  the  supply  of  potential  energy 
stored  up  in  the  chemical  compounds  of  the  hay  will  have 
become  nearly  or  completely  exhausted  —  transformed 
into  the  kinetic  form  and  expressed  in  the  life  activities  of  the 
plant  and  animal  population. 

Thus,  after  a  few  weeks,  the  hay  infusion  world  has  reached 
a  standstill  —  extermination  faces  the  population  and 
inevitably  occurs  unless  microscopic  green  plants,  possibly 
Sphaerella,  find  their  opportunity  to  develop  in  the  energy- 
exhausted  environment  and  proceed  to  entrap  the  kinetic 
energy  of  sunlight,  store  it  up  in  carbohydrates  and  proteins, 


METABOLISM  OF  COLORLESS  PLANTS     53 

and  thus  restore  energy  in  the  potential  form  to  the  hay 
infusion. 

If  this  occurs,  the  hay  infusion  world  is  a  microcosm 
indeed  —  green  plants,  colorless  plants,and  animals  gradually 
become  reciprocally  adjusted  so  that  a  self -perpetuating  con- 
dition of  practically  stable  equilibrium  subvenes;  in  other 
words,  what  is  termed  a  '  balanced  aquarium.'  The  cres- 
cendo and  diminuendo  of  teeming  populations,  made  possible 
by  the  rapidly  changing  environmental  conditions  which  the 
bringing  together  of  hay  and  water  initiated,  is  replaced  by 
an  apparently  harmonious  interdependence  of  organisms 
demanding  different  food  conditions,  such  as  we  are  familiar 
with  in  the  world  at  large. 


CHAPTER  VII 
THE  MULTICELLULAR  ORGANISM 

The  student  of  Nature  wonders  the  more  and  is  astonished 
the  less,  the  more  conversant  he  becomes  with  her  operations; 
but  of  all  the  perennial  miracles  she  offers  to  his  inspection, 
perhaps  the  most  worthy  of  admiration  is  the  development  of 
a  plant  or  animal  from  its  embryo.  —  Huxley. 

IT  has  been  pointed  out  that  all  organisms  consist  of  one 
free  living  cell  or  of  many  cells,  and  some  idea  has  been 
gained  of  unicellular  forms  from  Sphaerella,  Paramecium,  and 
the  Bacteria  which  were  selected  to  illustrate  various  types 
of  nutrition.  We  are  now  in  a  position  to  consider  the  origin 
and  organization  of  the  individual  in  the  METAZOA  and 
METAPHYTA,  as  the  multicellular  animals  and  plants  are 
sometimes  called. 

Every  individual,  with  exceptions  to  be  noted  later,  begins 
its  existence  as  a  single  cell  which  has  been  set  free  as  such 
from  the  parent;  or  which  has  been  formed  at  fertilization 
by  the  fusion  of  two  cells,  or  GAMETES,  each  typically  de- 
rived from  a  separate  parent  individual.  The  former  is 
known  as  UNIPARENTAL,  or  ASEXUAL,  reproduction  and  the 
latter  as  BIPARENTAL,  or  SEXUAL,  reproduction.  Both 
asexual  and  sexual  methods  are  widespread  among  plants  and 
animals,  frequently  alternating  in  regular  sequence  in  the 
same  species  to  give  what  is  termed  an  ALTERNATION  OF 

GENERATIONS. 

The  most  remarkable  fact  about  the  reproductive  cells  is  the 
inherent  power  of  each  to  develop  into  a  replica  of  the  parent 

54 


THE    MULTICELLULAE   ORGANISM 


55 


species  from  which  it  has  separated.  Both  the  spore  and  the 
zygote  (fertilized  egg)  are  set,  one  may  say,  to  go  through  a 
series  of  changes  which  transform  an  apparently  simple  cell 
into  an  obviously  complex  multicellular  plant  or  animal  with 
all  the  tissues  and  organs  characteristic  of  the  species.  It*  is 
important,  at  this  point,  to  review  the  general  method  by 
which  the  development  of  the  adult  is  accomplished. 

Briefly,  the  modus  operandi  of  development  is  cell  division 
accompanied  by  differentiation.  The 
spore  (asexual)  or  the  fertilized  egg 
(sexual)  by  a  succession  of  cell  divi- 
sions, termed  CLEAVAGE,  passes 
from  the  single-cell  stage  to  a  two- 
cell  stage  and  then,  with  more  or 
less  regularity,  to  four-cell,  eight- 
cell,  sixteen-cell  stages,  etc.  If  these 
cells  separated  after  each  divi- 
sion, the  same  general  condition 
would  obtain  here  which  has  been 
seen  in  the  Protophyta  and  Proto- 
zoa, where  each  organism  is  a  com- 
plete free-living  cell.  Or  again,  if 
cleavage  merely  resulted  in  a  group 
of  so  many  exactly  similar  cells,  there  would  arise  a  colony  of 
unicellular  individuals  rather  than  a  multicellular  organism. 
Such  colonial  forms  are,  in  fact,  numerous  among  the  lower 
plants  and  animals,  and  show  nearly  all  grades  of  complexity 
from  simple  associations  of  a  few  identical  cells,  as  for  example 
in  Spondylomorum,  to  groups  of  many  thousands  in  which 
some  of  the  individuals  are  specialized  for  certain  functions. 
(Fig.  17.)  Volvox  affords  an  instructive  example  of  the 
latter  condition.  The  majority  of  the  cells,  ten  thousand  or 
more,  which  form  the  relatively  large  spherical  colony  are 


FIG.  17.  —  A  simple  colony  of 
unicellular  organisms  (Spondylo- 
morum) each  of  which  carries  on 
all  the  functions  of  nutrition  and 
reproduction.  Highly  magnified. 
(From  Hegner,  after  Oltmanns.) 


56 


FOUNDATIONS   OF   BIOLOGY 


flagellated  individuals  each  of  which  lives  a  practically  in- 
dependent existence  in  organic  union  with  its  fellows.  The 
chief  contribution  of  each  of  these  cells  to  the  economy  of  the 


FIG.  18.  — Volvox  globator,  a  large  colony  of  flagellated  unicellular  or- 
ganisms in  which  the  various  cells  have  become  organically  connected, 
and  certain  cells  specialized  for  reproduction.  A,  mature  colony  (highly 
magnified)  showing  sperm,  $  ,  and  eggs,  $  ,  in  various  stages  of  devel- 
opment. B,  four  cells  (more  highly  magnified)  showing  the  connections 
between  three  'somatic'  cells,  and  the  early  differentiation  of  a  repro- 
ductive cell,  rp;  cv,  contractile  vacuole;  st,  'eyespot'or  stigma.  (From 
Hegner,  after  Kolliker.) 

whole  results  from  the  lashing  of  its  flagella,  which  helps  to 
propel  the  colony  through  the  water.  But,  under  certain 
conditions,  some  of  the  cells  become  specialized  for  repro- 
duction and  form  new  colonies  which  sooner  or  later  are  set 


THE   MULTICELLULAR   ORGANISM  57 

free.  Thus  we  have  a  foreshadowing  of  that  differentiation 
and  physiological  division  of  labor  between  cells  which  is 
the  most  characteristic  feature  of  the  Metaphyta  and 
Metazoa.  (Figs.  18,  115.) 

However,  in  the  developing  multicellular  organism  cleav- 
age results,  sooner  or  later,  in  a  body  composed  of  cells  ^ 
which  possess  differentiations  of  one  kind  or  another  that 
adapt  them  for  the  special  part  they  are  destined  to  play  in 
the  economy  of  the  individual.  Thus  cell  division,  involving 
differentiation,  is  the  keynote  of  development  in  the  higher 
plants  and  animals. 

Among  animals,  for  example,  the  cells  which  arise  from 
the  cleaving  egg  frequently  become  arranged  so  that  they 
form  the  surface  of  a  hollow  sphere  of  cells  known  as  a  BLAS- 
TULA.  All  the  cells  at  first  appear  essentially  similar,  but 
soon  those  at  one  side  of  the  blastula  become  invagi- 
nated  until  the  central  cavity,  termed  the  BLASTOCOEL,  is 
largely  obliterated.  Accordingly  there  results  the  GASTRULA 
stage,  which  may  be  roughly  compared  to  a  sack,  with  an 
opening  to  the  exterior  termed  the  BLASTOPOBE,  composed  of 
two  layers  of  cells.  The  outer  layer  is  known  as  the  ECTO- 
DERM, and  the  inner,  which  lines  the  gastrula  cavity  (ENTERIC 
CAVITY),  as  the  ENDODERM.  The  ectoderm  comprises  cells 
which  are  already  somewhat  differentiated  among  themselves 
for  special  purposes,  but  which,  as  a  whole,  form  a  primary 
tissue  with  general  functions  of  its  own,  chiefly  sensory  and 
locomotor.  Similarly  the  endoderm  consists  of  cells  which, 
as  a  group,  form  the  nutritive  cells  of  the  embryonic  animal. 
(Fig.  19.) 

In  the  gastrula  stage  of  most  animals,  a  third  layer  of  cells 
arises  typically  from  the  endoderm  and  becomes  disposed 
between  the  ectoderm  and  endoderm.  This  new  middle 
layer  is  the  MESODERM.  In  this  way  the  so-called  three 


58 


FOUNDATIONS    OF   BIOLOGY 


PRIMARY  GERM  LAYERS  are  established  which  are  characteris- 
tic of  the  developing  animal,  and  from  these  the  specialized 
tissues  which  compose  the  various  systems  of  organs  of  the 


I) 


G 


II 


— b 


FIG.  19.  —  Early  stages  in  the  development  of  the  egg  of  a  Sea  Urchin.  A-F,  cleavage 
and  formation  of  the  blastula;  G,  section  of  blastula  showing  the  beginning  of  gastrula- 
tion;  H-I,  early  and  later  gastrula  stages,  a,  ectoderm;  b,  endoderm;  c,  blast ocoel; 
d,  blastopore,  leading  into  the  enteric  cavity;  e,  cells,  arising  from  the  endoderm, 
destined  to  form  the  mesoderm. 

adult  are  derived.  For  example,  the  ectoderm  by  cell  divi- 
sion and  differentiation  gives  rise  to  the  outer  skin  and  central 
nervous  system;  the  mesoderm  to  muscular,  connective,  and 
supporting  tissues  and  the  blood  vascular  system;  while  the 


THE    MULTICELLULAR   ORGANISM 


59 


endoderm  forms  the  layer  of  cells  which  lines  the  alimentary 
canal  of  the  adult  organism. 

This  grouping  of  more  or  less  similar  cells  into  functional 
systems,  or  tissues,  is  at  the  basis  of  the  architecture  of 
multicellular  organisms,  and  thus  we  have  now  reached 
another  level  in  the  analysis  of  their  structure.  Although 
the  unit  of  organization  is  the  cell,  these  are  associated  in 
groups,  or  tissues,  which  represent  a  morphological  unit  of  a 


FIG.  20.  —  Portion  of  a  cross  section  of  the  small  intestine  of  the  Frog, 
highly  magnified  to  show  cellular  differentiation  and  tissues.  bl,  blood 
vessels;  aj,  unicellular  glands;  ep,  ordinary  absorptive  epithelial  cells 
lining  the  intestine;  es,  connective  tissue;  m.c.,  circular  muscle  cells; 
TO.  L,  longitudinal  muscle  cells;  pe,  peritoneum.  (From  Holmes,  after 
Howes.) 

higher  order.  A  TISSUE  may  be  defined  as  a  group  of 
essentially  similar  cells  specialized  to  perform  a  certain  func- 
tion. Examples  are  bone,  muscle,  and  nerve  in  animals; 
and  wood  and  bark  in  plants.  (Figs.  20,  21.) 

Since  the  similar  cell  components  of  multicellular  organisms 
are  grouped  to  form  tissues,  it  follows  that  the  major  working 
units,  or  ORGANS,  of  the  animal  or  plant  body  as  a  whole  are 
formed  of  tissues.  In  other  words  an  organ  is  a  complex  of 


60  FOUNDATIONS   OF   BIOLOGY 

tissues  which  has  assumed  a  definite  form  for  the  perform- 
ance of  a  certain  function:  for  example,  the  human  hand 
composed  of  bone,  muscle,  nerve,  etc.;  or  the  plant  leaf  with 
its  chlorophyll-bearing  tissue,  epidermal  covering,  etc. 

As  one  would  naturally  expect,  among  the  lowest  Meta- 
phyta  and  Metazoa  there  are  forms  in  which  the  body  is 
relatively  simple,  without  highly  specialized  tissues  and 
organs,  but  in  most  animals  specialization  is  carried  still 
another  step  forward  by  the  grouping  of  organs  devoted  to 


FIG.  21.  — Portion  of  a  cross  section  of  a  young  plant  stem,  magnified 
to  show  cellular  differentiation  and  tissues.  ca,  cambium;  co,  cortex;  e, 
epidermis;  p,  pith;  ph,  phloem;  x,  xylem.  (From  Gager.) 

the  performance  of  some  one  general  function  into  an  ORGAN 
SYSTEM.  An  animal  has  many  muscles,  each  of  which  is  an 
organ  but  which  collectively  constitute  a  working  unit,  the 
muscular  system;  or  it  has  stomach,  intestine,  liver,  etc.,  form- 
ing the  digestive  system.  On  the  other  hand,  even  in  the 
highest  plants,  differentiation  has  proceeded  neither  in  just 
the  same  way,  nor  so  far,  since  the  body  is  composed  of 
TISSUE  SYSTEMS  rather  than  organ  systems.  This  point  will 
be  clear  when  the  structure  of  the  plant  and  animal  body 
has  been  considered. 


CHAPTER  VIII 


THE  PLANT  BODY 

The  evidence  seems  to  show  beyond  question  that  our  present 
species  of  plants  have  descended  by  gradual  evolution  from 
simpler  and  fewer  species  which  formerly  existed  —  back,  it 
is  possible,  to  a  single  kind  which  throve  in  remotest  antiquity. 
—  Ganong. 

NEARLY  all  stages  exist  between  the  simplest  unicellular 
plant  body  such  as  is  exhibited  by  Sphaerella,  and  the  highly 
complex  condition  which  obtains  in  the  familiar  FLOWERING 
PLANTS,  technically  known  as  SEED  PLANTS  or  SPERMA- 
TOPHYTES.  A  simple  type  is  found  among  the  filamentous 
green  Algae  commonly  called  pond  scums.  In  forms  such  as 


>*"  -"•--, 


FIG.  22.  —  Spirogyra.     Portion  of  a  filament,  highly  magnified,  showing  cell  wall, 
cytoplasm,  nucleus,  vacuole  of  cell  sap,  and  spiral  chloroplastid.      (From  Coulter.) 

Spirogyra  the  body  of  the  plant  consists  of  a  series  of  similar 
cells  placed  end  to  end  and,  therefore,  from  one  point  of  view, 
may  be  looked  upon  as  a  colony  of  cells  since  the  individual 
cells  of  the  filament  are  essentially  independent.  (Fig.  22.) 
In  Ulothrix,  another  simple  form,  the  filament  instead  of 
floating  free  is  attached  by  a  more  or  less  specialized  cell 
devoid  of  chlorophyll.  (Fig.  49,  A.)  Still  more  common 
are  plant  bodies  composed  of  branching  filaments  of  cells. 
The  branches  may  all  be  similar,  or  there  may  be  a  chief  axis 

61 


62 


FOUNDATIONS    OF   BIOLOGY 


with  lateral  branches  of  different  form.  Frequently  the 
branches,  though  still  composed  merely  of  filaments  of  cells 
placed  end  to  end,  may  show,  for  example,  larger  chloro- 
plastids  and  thus  be  more  active  in  photosynthesis.  This  is 
essentially  the  same  general  division  of  labor  that  occurs 

between  the  stem  and 
leaves  of  higher  plants, 
but  without  the  attend- 
ant structural  differen- 
tiation of  the  parts  into 
tissues.  (Cf.  p.  413.) 

The  next  specializa- 
tion we  find  is  in  regard 
to  the  character  of  the 
growth.  Whereas,  in 
simple  unbranched  fila- 
mentous forms,  growth 
takes  place  as  a  rule  by 
the  division  of  all  of  the 
cells  composing  it,  in 
the  branched  types  this 
is  usually  restricted  to 
one  or  more  cells  near 
the  end  of  each  filament. 
Thus,  depending  on  the 
character  of  the  growth 
from  the  apical  cells,various  complex  forms  of  massive  branch- 
ing structures  arise  as,  for  instance,  in  many  of  the  Red 
Seaweeds.  In  these  plants  the  chloroplastids  are  chiefly  de- 
veloped in  the  cells  on  the  surface;  which  again  indicates  the 
physiological  division  of  labor  referred  to  above  and  suggests 
that  many,  if  not  all,  of  the  modifications  of  the  simple  plant 
bodies  thus  far  considered  are  provisions  for  the  purpose  of 


FIG.  23.  —  A  common  Seaweed  (Fucus).  One 
of  the  Brown  Algae,  showing  comparatively  sim- 
ple thallus  structure.  (From  Coulter.) 


THE    PLANT   BODY 


63 


bringing  about  the  most  favorable  exposure  to  light  of  the 
photosynthetic  apparatus. 

Another  method  of  attaining  the  same  object  is  found  in 
other  Seaweeds.  In  the  common  Sea  Lettuce  (Ulva)  and 
the  Rockweed  (Fucus)  the  plant  body  takes  the  form  of  a 
plate  of  cells,  as  a  result  of  cell  division  occurring  in  two 
planes,  and  then  this  THALLUS  usually  becomes  thicker  by 
division  of  the  cells  in  a  third  plane  also.  As  a  result  of 
further  modifications  of  the  thallus,  the  single  attaching  cell 
of  the  simple  filamentous  types  is  replaced  in  the  larger  Sea- 


FIG.  24. — The  Giant  Kelp.  A  marine  Alga 
which  may  attain  more  than  200  feet  in  length. 
A  thallus  plant  exhibiting  distinct  leaf-like  and 
stem-like  structures,  and  holdfast.  (From 
Ganong.) 


weeds  by  massive  HOLDFASTS  which  anchor  them  securely 
to  rocks.  Still  there  is  no  marked  differentiation  in  the 
cellular  components  of  the  holdfasts  because  they  perform 
only  this  one  function  of  the  roots  of  higher  forms;  the  ab- 
sorption of  food  materials  dissolved  in  the  water  being 
carried  on  by  the  individual  cells  of  the  whole  plant.  Al- 
though among  the  most  complex  Seaweeds,  for  example  in 
the  Kelps  and  Gulf  weed,  the  form  of  the  thallus  is  highly 
modified  into  divisions  which  serve  certain  of  the  func- 
tions of  root,  stem,  and  leaf  of  higher  plants,  still  none  of 
the  fundamental  tissue  differentiations  so  characteristic  of 
the  higher  forms  occur.  Similarity  of  function  has  given 
rise  to  ANALOGOUS  structures.  (Figs.  23,  24,  25.) 


64  FOUNDATIONS    OF   BIOLOGY 

It  is  among  the  so-called  VASCULAR  PLANTS  —  the  Ferns 
and  Flowering  Plants  —  that  the  most  highly  specialized 
plant  body  occurs.  As  will  be  explained  in  more  detail  later, 
these  plants  exhibit  in  their  life  history  an  alternation  of 
generations:  a  sexual  plant  (GAMETOPHYTE)  bearing  gametes 
gives  rise  to  a  non-sexual  spore-bearing  plant  (SPOROPHYTE) 


FIG.  25.  —  Gulfweed  (Sargassum)  showing  the  'stem,'  'leaves,'  and 
the  berry-like  floats  of  the  thallus.  (From  Coulter,  Barnes,  and 
Cowles.) 


which  in  turn  produces  a  gametophyte.  These  two  genera- 
tions exhibit  marked  differences  in  structure.  The  game- 
tophyte body  is  relatively  very  simple,  consisting  merely 
of  a  few  cells,  the  main  function  of  which  is  to  develop  male 
and  female  gametes.  On  the  other  hand,  the  sporophyte  is 
composed  of  a  number  of  specialized  tissues  and  organs,  and 
is  the  conspicuous  generation  which  is  recognized  by  everyone 
as  a  'Fern'  or  a  'Flowering  Plant.' 


THE   PLANT   BODY 


65 


A.  GROSS  STRUCTURE 

The  body  of  the  sporophyte  of  a  typical  Flowering  Plant  is 
clearly  differentiated  into  two  parts,  ROOT  and  SHOOT.  The 
root  is  the  organ  of  attachment,  as  well  as  of  absorption  of 
food  material  in  solution.  The  shoot  consists  of  STEM  and 
LEAVES.  The  stem  is  largely  a  passive  structure  and  forms 
the  connecting  link  between  the  root  and  the  photosynthetic 


FIG.  26.  —  A,  tap  root  of  the  Dandelion;  B,  fibrous  roots  of  a  grass;  C,  clustered  and 
fleshy  roots  of  the  Dahlia.     (From  Bergen  and  Davis.) 

apparatus  of  the  leaf.    The  reproductive  organs  (SPORANGIA) 
are  usually   developed   as   appendages   of  modified  leaves 

(SPOROPHYLLS) . 

1.   Root 

The  PRIMARY  root  of  a  young  plant,  which  is  usually  a 
continuation  downward  from  the  shoot,  may  persist  through- 
out the  life  of  the  plant  as  the  chief  root  and  merely  give  off 
laterally  small  secondary  roots.  Such  a  root  system,  known 
as  a  TAP  root,  is  common  in  many  herbs,  as  for  example  the 
Dandelion.  More  often  the  primary  root  is  entirely  replaced 


66 


FOUNDATIONS    OF   BIOLOGY 


by  the  SECONDARY  roots  which  radiate  and  branch  in  all  direc- 
tions from  the  main  axis  of  the  plant  until  they  form  a  com- 
plex underground  structure.  This  may  equal  in  size  the 
part  of  the  plant  body  which  is  developed  above  the  surface 
by  the  shoot  system. 

In  plants  which  live  through  two  years  (BIENNIALS)  ,  often 


Spring  Seedling 

germination      growth 


Seed  (winter  rest) 


Adult  plant 


FIG.  27.  —  The  seasonal  history  of  an  annual  plant,  a  Bean. 
(From  Densmore.) 

the  first  year  is  spent  in  storing  up  food.  Sometimes  this  is 
in  the  roots,  in  which  case  they  are  greatly  enlarged  to  form 
a  reservoir  of  material,  at  the  expense  of  which  during  the 
second  season  the  plant  rapidly  develops  flowers  and  seeds. 
These  storage  roots  may  be  tap  roots  as  in  the  Turnip,  or 
lateral  roots  as  in  the  Dahlia  and  Sweet  Potato.  (Figs.  26, 27, 28.) 
Although  the  contact  of  the  plant  with  its  environment 
through  its  roots  is  ordinarily  underground,  tropical  plants 
in  particular  frequently  develop  AERIAL  roots  from  the  stem  or 


THE    PLANT   BODY 


67 


First  summer 
(Photosynthesis 
and  storage) 


Second  Summer 
(Photosynthesis 
and  reproduction) 


Second  winter 
(Death) 


FIG.  28. — The  seasonal  history  of  a  biennial  plant,  White  Sweet  Clover  (Melilotus). 
(From  Densmore.) 

its  branches.  Roots  which  rise  from  such  unusual  places  are 
called  ADVENTITIOUS  roots.  In  certain  species  the  aerial 
roots  hang  free  in  the" air  and  absorb  moisture  from  the  at- 
mosphere. In  addition,  such  roots 
may  develop  chlorophyll  and  so 
perform  the  characteristic  func- 
tion of  leaves.  In  the  Fig  tree  the 
aerial  roots  grow  from  the  branches 
down  to  the  earth  where  they 
become  attached  and  eventually 
form  a  stout  trunk  which  functions 
as  a  stem.  Comparable  to  these 
roots  are  the  so-called  Burxsess 
roots  of  some  Palms  and  of  the 
familiar  Indian  Corn.  (Fig.  29.) 

Many  plants  depend  chiefly  or      FIG  29._English  Ivy>  showing 
entirely  on  other  plants  for  their  the  aerial  roots  which  enable  it  to 

,.        -.  •    i  mi  r  i       clinS    to    walls.      (From    Ganong, 

lOOd  materials.      The  rOOtS  Of  SUch    aftcr  LeMaout  and  Decaisne.) 


68 


FOUNDATIONS   OF   BIOLOGY 


parasitic  species  frequently  grow  into  the  tissues  of  the  host, 
and  become  more  or  less  modified  into  suckers,  or  HAUSTORIA. 


FIG.  30.  —  Dodder,  a  parasitic  Flowering  Plant,  entwined  about  the  stem  of 
its  host,  a  Golden  Rod.  A,  cross  section  of  stem  of  host  to  show  its  penetra- 
tion by  the  Dodder  roots  (haustoria).  C,  several  Dodder  seedlings  growing  in 
the  soil  before  attachment  to  a  host,  h,  stem  of  host;  I,  scale-like_leaves; 
r,  haustoria;  s,  seedlings.  (From  Bergen  and  Davis.) 

In  the  Dodder  and  Mistletoe,  the  haustoria  enter  the  tissues 
of  the  stem  of  the  host,  while  in  many  of  the  false  Foxgloves 
(Gerardia)  they  enter  the  tissues  of  the  roots.  In  some 


THE    PLANT   BODY 


69 


aquatic  and  parasitic  plants  roots  are  absent,  their  function 
being  taken  over  by  other  parts  of  the  body  such  as  stem  or 
leaves.  (Fig.  30.) 

Without  multiplying  examples,  it  is  clear  that  the  part  of 
a  plant  which  the  botanist  calls  a  root,  and  which  typically 
anchors  the  plant  to  the  earth  and  takes  water  with  food 
materials  in  solution  from  the  soil,  frequently  is  highly  modi- 


FIG.  31.  —  Propagation  of  the  Strawberry  plant  by  runners.  A,  parent  plant;  B,  young 
plant;  6,  modified  leaf;  r,  runner  or  stem.    (From  Bergen  and  Caldwell.) 

fied  and  even  assumes  the  duties  of  other  organs  in  certain 
plants  which  are  adapted  for  special  places  in  the  economy 
of  nature. 

2.   Stem 

The  stem  of  the  vascular  plants  is  the  axis  of  the  shoot  and 
has  two  primary  functions.  First,*  to  support  and  raise  the 
leaves  into  a  position  of  vantage  with  respect  to  light;  and, 
second,  to  act  as  the  medium  of  communication  between  the 
absorbing  organs,  or  roots,  and  the  photosynthetic  organs, 
or  leaves.  But,  like  the  root,  it  may  be  modified  and  diverted 


70 


FOUNDATIONS    OF   BIOLOGY 


from  its  typical  structure  and  take  over  more  or  less  of  the 
functions  of  other  parts. 

For  the  purpose  of  propagation,  creeping  stems  occur 
such  as  the  surface  RUNNERS  of  the  Strawberry,  and  the 
underground  RHIZOMES  of  many  Sedges,  Grasses,  and 
common  Ferns.  Sometimes  the  stem  to  a  large  extent  re- 
places the  root  system,  but  more  often 
it  acts  as  an  underground  reser- 
voir in  which  food  material  is  stored 
up  during  the  short  growth  period  for 
the  rapid  development  of  the  flower- 
ing shoot.  This  is  well  seen  in  some 
of  the  early  spring  Flowering  Plants  of 
New  England  such  as  Bloodroot  and 
Trillium.  (Figs.  31,  39.) 

Again,  the  stem  is  greatly  short- 
ened to  form  a  BULB  or  a  CORM;  types 
particularly  common  in  plants  adapted 
to  dry  soil.  (Fig.  32.)  Extremely  arid 
regions  are  characterized  by  plants, 
such  as  the  Cacti,  in  which  the  leaves 
are  completely  suppressed  to  prevent 
rapid  evaporation ;  their  function  being 
taken  over  by  the  stem  which  is  pro- 
vided with  well-developed  chlorophyll- 
bearing  tissue.  Sometimes  parts  (branches)  of  the  stem 
may  superficially  resemble  a  leaf  by  being  flattened  or  other- 
wise modified,  as  in  the  Prickly  Pear,  the  apparent  leaves  of 
the  so-called  Smilax  (Myrsipmfllum) ,  and  the  filamentous 
'leaves'  of  Asparagus.  (Fig.  33.)  Finally,  the  versatility  of 
the  stem  is  illustrated  by  the  thorns  of  the  Honey  Locust, 
the  twining  tendrils  of  the  Grape,  and  the  tuber  of  the 
Potato  which  is  essentially  a  'concentrated  rhizome.' 


FIG.  32.— Bulb  of  a  Hya- 
cinth, in  section,  showing 
roots,  stem,  bases  of  leaves 
of  previous  year  stored  with 
food,  and  new  foliage  leaves 
about  the  flower  cluster. 
(After  Figurier.) 


THE    PLANT   BODY 


71 


FIG.  33.  — Stem  of  Smilax  (Myrsiphyllum).    d.  leaf-like  branch,  or  cladophyll,  situated 
in  the  axil  of  a  leaf;   I,  leaf;    ped,  flower  stalk.      (After  Bergen.) 


3.   Leaf 

Although  the  leaves  of  the  higher 
plants  exhibit  much  diversity  in 
form,  they  agree  in  their  essential 
features.  The  fundamental  func- 
tions of  leaves  are  to  expose  to 
the  sunlight  the  chlorophyll  appa- 
ratus and  to  afford  a  surface  for 
evaporation  and  the  exchange  of 
gases  with  the  environment.  Ac- 
cordingly the  principal  part  of  a 
typical  leaf  is  a  broad  blade,  or 
LAMINA,  which  affords  the  optimum 
conditions  for  exposure.  The  leaf 
is  usually  attached  to  the  stem  by 
a  stalk,  or  PETIOLE,  which  is  some- 


FIG.  34.  —  Leaf  of  a  Flowering 
Plant,  showing  blade,  or  lamina, 
petiole,  and  two  stipules  at  the 
leaf  base.  (From  Ganong,  after 
Gray.) 


72 


FOUNDATIONS   OF   BIOLOGY 


what  modified  at  the  point  of  union  with  the  stem  into  a 
LEAF  BASE  from  which  arise  leaf-like  appendages,  or  STIP- 
ULES. When  the  petiole  is  absent  the  lamina  of  such  a  sessile 
leaf  appears  to  arise  directly  from  the  stem. 
(Fig.  34.) 

The  leaf,  like  the  root  and  the  stem,  ex- 
hibits numerous  modifications  in  adaptation 
to  other  functions.  The  chlorophyll-bearing 
tissue  may  be  nearly  or  completely  sup- 
pressed, as  in  the  SCALES  which 
enclose  winter  buds  in  a  pro- 
tective case.  These  are  con- 
spicuously developed  in  the 
Horse  Chestnut  and  the  Hick- 
ory. (Fig.  35.)  Or  the  scale 
leaves,  in  addition  to  affording 
protection,  may  act  as  reser- 
voirs in  which  food  materials 
are  stored,  an  example  of 
which  is  the  familiar  Onion. 
(Fig.  36.)  All  transitions  be- 
tween scale  leaves  and  typical 
foliage  leaves  may  frequently 

FIG.  35.  — Shoot     .  .  <•   i  T  i      r 

of  Horse  chestnut   be  seen  in  an  unfolding  leaf 


sea 


bud.      Still  more  marked  de- 


FIG.  36— Onion 
leaf,  cut  longitudi- 
nally, bl,  blade; 
int,  hollow  interior 
of  blade;  s,  thin 
sheath  of  leaf; 


showing    winter 
buds    enclosed    by 

thick  scale  leaves;   partures  from  the  usual  leaf- 

k,     small     axillary     r  ,i  e  <snrnp       sea,  thickened  base 

bud;  x,  scar  of  leaf    IOrm  are 

of  previous  season.    climbing   plants  such   as   the 

(From  Campbell.)  °     ' 

Sweet  Pea,  the  SPINES  of  the 
Barberry  and  the  Thistle,  and  the  'insect  traps'  of  Pitcher- 
plants  and  Sundews  which   capture  small  living  animals. 
(Figs.  37,  38.) 
Leaf  modification  in  another  direction  occurs  in  the  spore- 


of  leaf.  (From 
Bergen  and  Davis, 
after  Sachs.) 


THE   PLANT   BODY 


73 


FIG.  37.  —  Common  Pitcher-plant  (Sarracenia  purpurea).    At  the  right,  one 
of  the  pitcher-like  leaves  is  shown  in  cross  section.     (From  Bergen.) 


FIG.  38. —  Leaves  of  Sundew  during  digestion  of  captured  prey.  The  one  at 
the  left  has  all  the  tentacles  closed ;  the  one  at  the  right,  only  half  of  them  closed 
over  the  prey.  (From  Bergen.) 


74 


FOUNDATIONS   OF   BIOLOGY 


FIG.  39.— The  Sensitive  Fern  (Onoclea 
sensibilis),  showing  vegetative  leaf,  and 
spore  leaf,  or  sporophyll,  arising  from  the 
rhizome.  (From  Bergen  and  Davis.) 


bearing  structures  of  Ferns 
and  Flowering  Plants.  In 
some  Ferns  the  spore  cases 
(sporangia)  are  borne  upon 
typical  leaves,  while  in  others 
they  arise  on  special  leaves 
with  chlorophyll-bearing 
tissue  partly  or  completely 
suppressed.  Such  leaves 
which  are  given  over  to  the 
production  of  spores,  as  in 
the  Sensitive  Fern,  are  known 
as  SPOROPHYLLS.  (Fig.  39.) 
In  the  Flowering  Plants,  the 
FLOWER  is  a  group  of  sporo- 
phylls,  known  as  CARPELS 
and  STAMENS,  associated  in 
most  cases  with  certain  sterile 
leaf  structures,  termed  SE- 
PALS and  PETALS,  which  af- 
ford protection  to  the  sporo- 
phylls  and  offer  attraction  to 
insect  visitors.  (Figs.  40,  58.) 
We  shall  consider  the 
structure  of  the  flower  in 
more  detail  in  discussing  re- 
production in  plants,  but  it 
is  essential  now,  having  con- 
sidered the  fundamental  di- 
visions (root,  stem,  and  leaf) 
of  the  body  of  vascular 
plants,  and  some  of  the  adap- 
tive modifications  of  these 


THE    PLANT   BODY 

^Corolla 


75 


FIG.  40.  —  The  Floral  parts  of  the  Alpine  Azalea  (Loiseleuria).  Collectively 
the  sepals  constitute  the  calyx,  and  the  petals,  the  corolla.  The  pistil  repre- 
sents several  united  carpels.  (From  Bergen  and  Caldwell,  after  Miiller.) 

parts  that  fit  plants  for  different  modes  of  life,  to  obtain 
some  insight  into  the  tissue  organization,  or  HISTOLOGY,  of  a 
typical  plant. 

B.   HISTOLOGY 

As  we  have  seen,  the  functions  of  organisms  are  performed 
by  their  protoplasm  which  constitutes  the  structural  units,  or 
cells.  The  cells,  when  specialized  for  a  particular  duty  in 
the  economy  of  the  organism,  are  usually  associated  in  more 
or  less  homogeneous  groups,  or  tissues.  Tissues,  in  turn,  are 
grouped  to  form  tissue  systems  and  organs;  that  is,  major 
divisions  of  the  body  which  allow  the  tissues  and,  therefore, 
the  cells  devoted  to  a  special  function  to  play  their  part  under 
the  most  suitable  relations  to  internal  or  external  conditions. 
It  is  important,  however,  as  we  resolve  the  individual  plant 
(or  animal)  into  its  component  cells,  tissues,  or  organs,  not  to 
lose  sight  of  the  fact  that  these  parts  are  all  at  work  for  the 
good  of  either  the  individual  or  the  race.  The  many  dif- 
ferent kinds  of  work  which  are  being  carried  on  by  the  organ- 
ism, whether  it  is  simple  or  complex,  must  provide  in  the 
final  analysis  for  two  things:  the  support  or  nutrition  of  the 


76 


FOUNDATIONS   OF   BIOLOGY 


individual  and  the  production  of 
other  similar  individuals,  reproduc- 
tion. 

In  order  to  obtain  a  mental  picture 
of  the  essential  working  plan  of  the 
tissue  distribution  in  the  body  of  a 
higher  Flowering  Plant,  we  shall  con- 


FIG.  41 .  —  Ideal  vertical  section  through  a  generalized 
plant,  showing  the  typical  distribution  of  the  systems 
of  tissues.  The  central  cylinder,  or  stele,  comprises  the 
pith  (coarse  dotted),  xylem  (diagonally  lined),  phloem 
(cross  lined),  with  cambium  (fine  dotted)- between  and 
extending  to  growing  points  of  root  and  shoot.  The 
cortical  system  (crosses)  forms  the  'hollow'  cylinder 
which  surrounds  the  central  cylinder  and  is,  in  turn, 
enclosed  by  the  outer  hollow  cylinder  (double  lined), 
or  dermal  system.  (From  Ganong.) 


sider  first  an  ideal  vertical  section 
through  a  generalized  plant.  (Figs. 
41,  43,  a.) 

The  root  and  shoot  system  together 
constitute  a  continuous  body,  which 
may  be  regarded  as  forming  a  long 
narrow  cylinder  of  cells,  the  bottom 
of  which  is  the  growing  point  of  the 
primary  root,  and  the  top,  the  grow- 
ing point  of  the  shoot.  This  primary 
cylinder,  in  turn,  is  made  up  of  a  solid 
central  cylinder  of  cells,  surrounded 
by  two  concentric  'hollow1  cylinders. 
The  inner  of  these  concentric  hollow 
cylinders  surrounds  the  central  cyl- 
inder, and  in  turn  is  surrounded  by 


THE    PLANT   BODY  77 

the  outer  cylinder.  The  latter  forms  the  surface  of  the  prim- 
ary cylinder,  or  the  outer  layer  of  cells  of  the  plant.  These 
three  cylinders  comprise  the  primary  tissue  systems. 

The  central  cylinder,  known  as  the  STELE,  runs  continu- 
ously throughout  root  and  stem,  and  sends  bifurcations  into 
the  branches  by  which  certain  of  its  elements  enter  the 
leaves  to  form  the  VEINS.  It  provides  the  PITH,  or  primary 
axial  tissue,  and  the  VASCULAR  BUNDLES.  The  latter  include 
the  food-conducting  tissue  (PHLOEM),  the  water-conducting 
tissue  (XYLEM)  ,  and  between  them  the  actively  growing  tissue 


- WALL 

-PLASTID 
--SAF^CAVITY 


—  NUCLEUS 
J  NUCLEOLU9 
-  CYTOPLASM 


FIG.  42.  —  Optical  section  (highly  magnified)  of  a  generalized  plant  cell. 
(From  Ganong.) 

(CAMBIUM).  The  cambium  becomes  continuous  with  the 
tissues  at  the  growing  points  of  stem  and  roots,  and  together 
these  embryonic  tissues,  called  MERISTEM,  may  be  regarded 
as  the  growth  system  of  the  plant. 

The  hollow  cylinder  immediately  surrounding  the  solid 
central  cylinder  comprises  the  CORTICAL  system  which  pro- 
vides the  CHLORENCHYMA,  or  chlorophyll -bearing  tissue  of  the 
young  stem  and  of  the  leaves,  and  also  the  CORTEX  of  bark 
and  root. 

The  outside  cylinder  forms  the  DERMAL  system  which 
supplies  the  hair  layer  of  the  surface  of  the  young  root  and 
the  protective  EPIDERMIS  covering  the  stem  and  leaf. 


78  FOUNDATIONS   OF   BIOLOGY 

With  this  diagrammatic  arrangement  of  the  tissue  systems 
of  the  plant  in  mind,  we  are  in  a  position  to  consider  the 
histology  of  a  typical  root,  stem,  and  leaf  of  the  higher 
Flowering  Plants;  in  other  words,  to  resolve  the  cylinders  or 
tissue  systems  of  the  plant  into  their  component  parts  by 
the  study  of  transverse  and  longitudinal  sections  cut  at 
various  levels,  and  so  to  determine  the  general  character 
and  distribution  of  the  cells  as  seen  under  the  microscope. 

1.  Root 

An  examination  of  the  tip  of  a  root  shows  that  it  is  covered 
with  a  large  number  of  cells  which  form  the  ROOT  CAP. 
These  cells  are  gradually  rubbed  away  as  the  root  works 
through  the  soil  and  continually  replaced  by  new  ones  from 
the  GROWING  POINT  which  is  immediately  above.  The  nu- 
merous, small,  densely-packed  cells  constituting  the  growing 
point  represent  the  region  of  cell  formation  for  the  entire  root 
tip,  since  near  the  center  is  a  group  of  cells  from  which 
smaller  cells  are  divided  off,  and  these  in  turn  absorb  food 
materials  and  attain  the  normal  size.  It  will  be  recalled 
that  the  growing  point  is  continuous  with  the  cambium 
region  above,  and  it  thus  represents  the  growth  system 
(meristem)  at  the  root  tip.  (Fig.  43.) 

Just  above  the  growing  point  is  the  GROWTH  ZONE  which 
includes  cells  recently  formed  by  the  tip  in  its  growth  down- 
ward. In  this  region  the  cells  enlarge  rapidly,  especially  in 
length,  and  at  the  same  time  retain  relatively  thin  cell  walls. 
The  cytoplasm  of  these  cells,  by  the  development  and  coales- 
cence of  large  vacuoles  of  cell  sap  (water,  sugar,  and  other 
substances  in  solution) ,  soon  forms  merely  a  lining  closely  ap- 
plied to  the  wall ;  a  condition  characteristic  of  many  plant  cells 
in  contrast  with  those  of  animals.  In  the  growth  zone  also 
is  clearly  seen  on  the  surface  the  protective  layer,  or  epidermis, 


THE   PLANT   BODY 


79 


and,  just  within,  the  cortex  made  up  of  several  layers  of  cells. 
Still  further  toward  the  center  of  the  root,  the  central  cylinder 
appears,  which  shows  differentiating  vascular  bundles. 

Passing  now  to  a  point  a  little  above,  we  find  the  growth 
zone  merging  imperceptibly  into  a  region  in  which  many  of  the 


C 


FIG.  43.  —  Cell  division  and  tissue  differentiation  in  a  growing  root  tip.  Dia- 
gram of  root  (a)  shaded  to  show  the  several  regions  from  which  the  highly  mag- 
nified sections  (b,  c,  d,  d',  e)  are  taken.  (From  Densmore.) 

epidermal  cells  on  the  outer  surface  of  the  root  are  modified 
into  ROOT  HAIRS.  It  has  been  emphasized  above  that  the 
primary  function  of  the  root  is  the  intake  of  certain  of  the 
elements  of  food  in  solution.  This  function  is  performed 
almost  entirely  by  osmosis  through  the  extensive  area  af" 
forded  by  the  surface  of  the  root  hairs,  and  accordingly  cells 


80 


FOUNDATIONS   OF   BIOLOGY 


of  this  type  form  the  vital  point  of  contact  between  the  root 
and  its  environment.  The  root  hairs  exhibit  the  selective 
power  of  protoplasm  to  a  remarkable  degree.  For  example, 
Red  Clover  plants  and  Barley  plants  when  burned  yield  about 
the  same  proportion  of  mineral  matter  as  ash.  But  the 

Barley  ash  contains  nearly 
twenty  times  as  much 
silica  as  the  Clover,  while 
the  latter  contains  nearly 
six  times  as  much  lime  as 
the  Barley.  (Fig.  44.) 

In  the  zone  in  which 
root  hairs  are  present  the 
central  cylinder  of  the 
root  shows  still  more  cell- 
ular specialization.  The 
young  vascular  bundles 
are  differentiated  into  the 
phloem  tissue,  character- 
ized by  its  small  angular 
ducts;  while  the  develop- 
ing xylem,  with  its  large 

FIG.  44.  —  Root  hair  (very  highly  magnified)  ducts,  has   obliterated   the 
showing  its  relation  to  adjoining  cells  of  the  root  ...  , 

and  to  particles  of  the  soil,  a,  vacuole  filled  primitive  ground  tlSSUC,  Or 
with  cell  sap;  b,  cytoplasm  (dotted);  c  soil  ith  Between  the  Xylem 

particles;  d,  nucleus  within  the  cytoplasm  lining  * 

the  ceil  wail.  and   the  phloem  appears 

the  developing  cambium,  but  this  begins  its  characteristic 
growth  contribution  somewhat  above  the  hair  zone.  Indeed, 
as  we  pass  upward  from  the  region  where  the  root  hairs  are 
developed,  the  cellular  structure  becomes  more  and  more 
similar  to  that  of  the  stem;  the  older  woody  roots  of  trees 
and  shrubs  being,  from  the  standpoint  of  both  structure  and 
function,  stems. 


THE    PLANT   BODY 


81 


2.   Stem 

Just  as  cell  division  in  the  meristem  tissue  of  the  growing 
tip  of  the  root  proceeds  in  such  a  way  that  the  direction 
of  growth  is  typically  downward,  so  in  the  similar  region  at 
the  growing  point  (BUD)  of  the  shoot  (stem  and  leaves)  cell 
multiplication  results  in  progress  upward.  In  other  words, 
from  the  region  where  root  and  shoot  merge,  the  growth  of 
the  plant  is  in  opposite 
directions. 

The  general  character 
of  the  embryonic  tissue 
is  essentially  the  same 
in  both  the  root  and 
shoot  regions,  being 
composed  of  densely 
packed,  more  or  less 
cubical  cells  which  are 
completely  filled  with 
protoplasm.  At  this 
stage  the  cells  have  not 
become  modified  for 
special  functions  by  the 
formation  of  vacuoles  of  cell  sap,  or  in  other  ways.  But  all 
the  tissues  of  the  stem  are  derived  from  the  cells  of  the 
growing  point,  so  that  slightly  below  this  region  three  areas 
are  to  be  noted  in  which  differentiations  are  in  progress. 
These  result  in  the  formation  of  the  outer  cylinder  (epider- 
mis), the  intermediate  cylinder  (cortex),  and  the  central 
cylinder  (stele)  in  which  the  ground  tissue  (pith)  is  gradually 
encroached  upon  by  the  developing  vascular  bundles.  The 
general  arrangement  of  these  fundamental  tissue  systems 
may  be  seen  in  a  transverse  section  of  the  young  stem  of  a 
Bean  which  affords  an  excellent  working  plan  of  stem  anat- 


FIG.  45.  —  Portioft  of  a  cross  section  of  the  stem 
of  a  young  plant  (Ricinus).  ca,  cambium;  co,  cor- 
tex; e,  epidermis;  p,  pith;  ph,  phloem;  x,  xylem. 
(From  Gager.) 


82 


FOUNDATIONS   OF   BIOLOGY 


omy  in  one  of  the  main  divisions  (Dicotyledons)  of  Flower- 
ing Plants.     (Fig.  45.) 

3.  Leaf 

The  embryonic  cells  forming  the  growing  point,  or  bud,  of 
the  shoot  comprise,  as  we  have  seen,  the  fundamentals  of 
both  stem  and  leaves;  that  is  to  say,  stem  and  leaves  arise 
together  in  buds.  The  method  of  formation  of  stem  and 
leaves  is  well  seen  in  the  buds  of  a  common  water  plant, 


FIG.  46.  —  A  bud,  of  unusually  elongated  form,  of  Elodea  canadensis,  in 
exterior  view  and  section,  showing  the  development  of  leaves;  X  150. 
(From  Ganong,  after  Kny.) 

Elodea.  Here  the  rounded  end  of  the  stem  is  composed  of 
the  characteristic  embryonic  tissue.  The  rudiment  of  each 
individual  leaf  is  first  visible  on  the  surface  as  an  enlarged  cell 
which  by  division  and  differentiation  gradually  develops  into 
a  flat  projection  of  epidermal,  cortical,  and  vascular  tissue, 
constituting  the  fully  formed  leaf.  (Fig.  46.) 

Bearing  in  mind  that  the  leaf  is  the  chief  organ  for  the  in- 
take and  utilization  of  the  energy  of  sunlight  and  for  the 
interchange  of  gases,  it  is  evident  that  it  forms  the  second  of 
the  two  chief  points  of  contact  of  the  plant  with  its  sur- 
roundings. This  intimate  relationship  is  manifest  in  the  in- 


THE    PLANT   BODY 


83 


numerable  adaptations  in  the  form  of  leaves,  but  the  funda- 
mental structure  of  all  can  be  reduced  to  a  common  plan  which 
may  be  illustrated  by  a  transverse  section  of  a  leaf.  (Fig.  47.) 
The  essential  features  of  a  leaf  consist  of  upper  and  lower 
limiting  membranes  (epidermis)  which  are  continuous  at  the 
edges  of  the  blade,  and  thus  enclose  the  supporting  and  con- 
ducting tissues  consisting  of  vascular  bundles  (veins),  and 


FIG.  47. —  Cross  section  of  a  typical  leaf,  highly  magnified,  a,  air  spaces;  b,  vein;  e, 
e',  upper  and  lower  epidermis;  p,  palisade  layer  of  chlorenchyma;  s,  stoma;  sp, 
irregularly  arranged  'spongy'  chlorenchyma  cells.  (From  Bergen  and  Davis.) 

the  chlorophyll-bearing  cells  (chlorenchyma)  which  carry  on 
the  work  of  photosynthesis. 

The  walls  of  the  epidermal  cells  are  impervious  to  water 
and  gases,  and  therefore  the  epidermis  is  perforated  with  tiny 
pores  (STOMATA)  which  lead  into  air  spaces  among  the  chlo- 
renchyma cells.  It  is  estimated  that  the  number  of  stomata 
in  the  epidermis  of  common  leaves  averages  about  500  per 
square  millimeter,  so  there  is  ample  provision  for  the  exchange 
of  water  vapor,  carbon  dioxide,  and  oxygen  with  the  atmos- 
phere. Each  stoma  is  enclosed  by  two  specialized  epidermal 
cells,  termed  GUARD  CELLS,  which  regulate  the  size  of  the 
opening  according  to  varying  internal  and  external  conditions. 


84  FOUNDATIONS   OF   BIOLOGY 

The  veins,  as  we  have  seen,  are  merely  the  extensions  into 
the  leaf  of  the  chief  elements  of  the  vascular  bundles  of  the 
stem.  They  form  the  framework  of  the  leaf,  as  well  as  the 
system  of  ramifying  highways  for  the  transportation  of 
materials  between  the  blade  as  a  whole  and  the  stem. 
In  cross-section  the  larger  veins  show  the  essential  features 
of  the  vascular  bundles  seen  in  the  stem,  lacking,  however, 
the  cambium. 

The  chief  tissue  of  the  leaf  is  the  chlorenchyma,  consisting 
of  chlorophyll-bearing  cells.  Immediately  under  the  upper 
epidermis  these  cells  are  arranged  in  a  definite  layer  known  as 
the  PALISADE  LAYER.  Below  this  region  the  cells  are  more  or 
less  irregularly  disposed  so  that  there  are  larger  and  smaller 
AIR  SPACES  between  them.  These  air  spaces  form  a  prac- 
tically continuous  system  of  passages  and  thereby  facilitate 
the  interchange  of  oxygen,  carbon  dioxide,  and  water  vapor 
between  the  leaf  cells  and  the  outer  world  through  the 
stomata. 

The  cytoplasm  within  the  thin  walls  of  the  chlorenchyma 
cells  forms  merely  a  lining  in  which  are  situated  the 
nucleus  and  numerous  specialized,  disc-shaped,  cytoplasmic 
bodies,  the  chloroplastids,  which  bear  the  chlorophyll  and 
therefore  appear  green.  It  will  be  recalled  that  these  are  the 
essential  agents  of  photosynthesis.  The  center  of  the  cell 
is  occupied  by  a  large  vacuole  of  cell  sap.  This  sap  is  usually 
under  considerable  pressure,  which  accounts  for  the  close 
application  of  the  cytoplasm  to  the  inside  of  the  cell  wall  and 
produces  the  turgor  characteristic  not  only  of  the  chloren- 
chyma cells,  but  also  of  many  other  types  of  plant  cells  as  well. 

C.   PHYSIOLOGY 

We  have  now  outlined  the  essential  structure  of  a  general- 
ized Flowering  Plant,  with  the  exception  of  the  parts  modi- 


THE    PLANT   BODY  85 

fied  for  reproduction.  Before  turning  to  the  flower  which 
in  its  function  has  to  do  with  the  race  rather  than  the 
individual,  it  is  important  to  consider  the  organism  as  a 
whole  —  how  the  various  cells,  tissues,  and  organs  cooperate 
in  the  nutrition  of  the  living  plant;  for  nutrition,  it  will  be 
recalled,  is  the  function  of  primary  importance  to  the  indi- 
vidual. 

The  essentials  of  nutrition  were  readily  described  in  the 
simple  green  plant  Sphaerella,  because  the  whole  organism 
comprises  but  a  single  cell  which  directly  interchanges  matter 
and  energy  with  its  environment.  But  with  the  establish- 
ment of  the  complex  plant  body,  an  organization  of  many 
millions  of  highly  specialized  cells,  the  intricate  interrelation- 
ship of  these  various  parts  to  the  nutrition  of  the  whole  — 
the  mechanical  engineering  of  the  plant  —  becomes  a  problem 

in  itself. 

1.    Circulation  Paths 

The  green  plant,  as  we  know,  takes  in  the  raw  materials 
and  builds  them  up  into  its  foodstuffs.  In  the  case  of  the 
higher  plants,  water  in  large  amounts  is  taken  in  through  the 
root  hairs.  Dissolved  in  this  water  are  various  substances  — 
nitrates,  phosphates,  sulfates,  etc.  —  supplying  most  of  the 
elements  that  are  necessary  for  the  make-up  of  protoplasm. 
The  leaves  admit  carbon  dioxide  through  the  stomata.  Thus 
the  substances  which  are  to  be  built  up  into  foodstuffs  enter  at 
the  opposite  ends  of  the  plant,  and  must  be  brought  together 
in  a  chemical  laboratory,  as  it  were,  in  order  that  their  union 
may  be  effected.  The  organ  in  which  food  construction  takes 
place  is  the  leaf,  and,  specifically,  in  the  chloroplastids  of  the 
chlorenchyma  cells.  Accordingly  we  must  consider  the  high- 
ways which  bring  the  raw  materials  from  the  root  hairs  to 
the  leaves  and  those  which  distribute  the  finished  products 
to  the  various  parts  of  the  plant  for  their  use.  (Fig.  48.) 


86  FOUNDATIONS   OF   BIOLOGY 

The  water  which  enters  the  root  hairs  is  given  a  start  up 
the  stem  by  'root  pressure'  due  to  osmotic  phenomena  in  the 
multitude  of  cells  of  the  root.  This  pressure  in  an  actively 
growing  tree  in  spring  may  be  nearly  forty  pounds  to  the 
square  inch.  Passing  from  the  xylem  of  the  root  the 
ascending  water  enters  the  similar  region  of  the  stem.  Here 
the  conducting  vessels  are  of  two  kinds:  namely,  greatly 
elongated  single  cells,  known  as  TRACHEIDS,  and  DUCTS. 
The  latter  are  really  tubes  which  have  been  formed  by  the 
absorption  of  the  contiguous  walls  of  many  long  cells  ar- 
ranged end  to  end.  The  mature  tracheids  and  ducts,  though 
originally  derived  from  living  cells,  are  devoid  of  protoplasm, 
and  form  a  series  of  non-living  tubes  extending  up  the  stem 
to  the  leaves,  through  which  they  are  distributed  in  the  veins. 
This  is  the  fluid-conducting  highway  from  root  to  leaf. 

The  supply  of  carbon  which  the  plant  needs  is  obtained 
from  carbon  dioxide  which  enters  the  leaves  through  the 
stomata.  The  water,  containing  various  salts  in  solution, 
which  has  been  taken  in  by  the  roots,  meets  the  carbon  diox- 
ide in  the  chlorenchyma  cells;  and  it  is  here  that  these 
raw  materials  are  manufactured  into  food.  Therefore,  the 
leaves  are  the  organs  specialized  for  assembling  the  materials 
of  the  inorganic  world  and  forming  from  them  new  chemical 
compounds  of  such  a  character  that  they  can  be  utilized  as 
building  material  for  the  plant  body  and  as  sources  of  energy 
for  carrying  on  the  vital  functions.  In  other  words,  the  new 
compounds  are  the  food  of  the  plant.  As  we  know,  through 
the  radiant  energy  of  light  a  complicated  series  of  chemical 
reactions  are  initiated  by  which  carbon,  oxygen,  and  hydrogen 
are  united  to  form  a  sugar.  In  this  process,  free  oxygen  is 
evolved  which  may  be  used  in  respiration  or  liberated  through 
the  stomata.  Part  of  the  sugar  thus  formed  is  directly  util- 
ized by  the  plant  as  fuel,  and  part  is  employed  as  the  basis  for 


THE   PLANT   BODY 


87 


the  manufacture  of  proteins  and  the  living  material  itself  by 

the  addition  of  nitrogen  and  various  other  chemical  elements. 

In  every  case  all  types  of  food  built  up  in  the  leaves  must 

be  distributed  to  the  organism  as  a  whole.  This  occurs 


Transpiration  Respiration  Photosynthesis 


Foods 

Cortex 

— \Phloem  (food  path) 
— \XyUm  (water  path) 
Pith 


Absorption 
Water 

Salts-'"^''' 
Oxygen' 


\Respiratfon 


FIG.  48. —Diagrammatic  presentation  of  the  chief  physiological  activities  of  a  Flower- 
ing Plant  (Bean).  The  first  leaves  (seed  leaves,  or  cotyledons)  are  richly  stored  with 
food  but  contribute  only  slightly  to  photosynthesis.  (From  Densmore.) 

chiefly  by  diffusion  in  the  form  of  soluble  carbohydrates  (e.g., 
grape  sugar)  or  in  soluble  nitrogenous  form  (e.g.,  amines  or 
soluble  proteins)  to  the  smallest  veinlets  and  then  on  to 
larger  and  larger  veins,  which  finally  deliver  it  to  the  stem. 


88  FOUNDATIONS   OF   BIOLOGY 

In  the  stem,  the  course  taken  by  the  food  depends  upon  the 
immediate  needs  of  the  plant.  It  may  pass  either  up  or  down 
through  the  phloem,  or  some  may  be  transferred  to  the  xylem 
and  carried  to  the  growing  tip  or  the  developing  flower  and 
fruit  for  immediate  use.  When  growth  is  not  active,  most 
of  the  food  passes  downward  through  the  phloem  to  supply 
the  cambium  and  to  be  stored,  chiefly  as  starch,  in  stem  and 
root.  In  brief,  all  the  living  cells  of  the  plant  directly  or 
indirectly  draw  upon  the  supply  of  food  circulating  through 
the  phloem,  so  we  may  look  upon  the  phloem  as  primarily  a 
food-distributing  system  from  the  leaf,  just  as  we  have  seen 
that  the  xylem  is  the  system  for  carrying  water  and  solutes 
from  root  to  leaf.  The  raw  materials  pass  up  through  the 
wood  and  the  finished  products  pass  down  through  the  bark. 

2.  Dynamics  of  Circulation 

The  question  will  naturally  arise  in  the  mind  of  the  reader: 
what  is  the  force  which  brings  about  the  circulation  of  all 
these  fluids  in  the  plant  body?  We  have  mentioned  that 
water  containing  solutes  enters  the  root  hairs  and  passes  to  the 
cortical  cells  and  ducts  of  the  xylem  by  the  physical  process 
known  as  osmosis.  It  is  now  believed  that  osmosis  in  the 
leaves  draws  water  from  the  ducts  of  the  stem  into  the  cells 
of  the  leaf.  It  is  also  thought  that  osmotic  forces  operating 
in  leaf  cells  are  adequate  to  lift  the  water  to  the  tops  of  the 
tallest  trees,  where,  in  turn,  the  water  is  removed  from  the  leaf 
cells  by  evaporation  through  the  stomata. 

The  outgo  of  water  by  evaporation  is  termed  TRANSPIRA- 
TION, and  is  brought  about  by  heat  energy  from  the  sur- 
rounding atmosphere.  In  the  last  analysis,  if  the  explanation 
suggested  above  is  correct,  the  energy  of  heat,  resulting  in 
evaporation  from  the  leaves,  is  chiefly  responsible  for  the 
movement  of  the  column  of  water  which  is  continually  pass- 


THE    PLANT    BODY  89 

ing  through  the  plant — entering  the  root  with  various  sub- 
stances in  solution  and  emerging  through  the  stomata  as 
water  vapor.  The  fact  that  much  more  water  usually  is 
evaporated  from  a  forest  than  from  an  equal  area  of  a  lake, 
affords  some  conception  of  the  part  played  by  vegetation  not 
only  in  returning  water  to  the  atmosphere  but  also  in  'con- 
suming' heat  energy  —  cooling  the  summer  air.  The  dynam- 
ics of  the  circulation  through  the  xylem,  however,  are  prob- 
ably by  no  means  so  simple  as  might  appear  from  the  theory 
just  outlined;  and,  moreover,  there  is  no  satisfactory  explana- 
tion of  the  causes  of  food  distribution  in  the  phloem,  further 
than  that  osmotic  phenomena  play  an  important  role. 

3.   Food  Utilization 

The  food  which  the  plant  has  constructed  and  distributed 
to  the  various  parts  of  its  body  must  be  employed  by  the 
individual  cells  in  supplying  the  material  and  energy  for  their 
life  processes.  It  is  important  not  to  lose  sight  of  the  cell  in 
the  larger  organization  of  which  it  is  a  part,  for,  in  the  final 
analysis,  the  life  of  the  individual  plant  is  but  the  life  of  the 
multitude  of  units  which  cooperate  toward  its  make-up. 
Although  the  cells  suppress  their  individuality  in  the  cor- 
porate whole  which  they  form,  the  life  of  the  plant  is  as  truly 
the  life  of  the  protoplasmic  units  which  form  it  as  is  the  life 
of  a  human  community  resident  in  the  individual  citizens. 

The  cells  select  from  the  food  stream  not  only  the  materi- 
als essential  for  their  individual  life,  but  in  addition  those 
which  they  require  for  the  performance  of  their  particular  part 
in  the  economy  of  the  whole.  But  doing  this  implies  work, 
and  work  means  expenditure  of  energy  —  the  same  energy  of 
sunlight  which  was  stored  in  the  food  during  its  construction 
by  the  chlorenchyma  cells  of  the  leaf.  In  order  to  release 
this  energy  RESPIRATION  must  occur.  Carbohydrates,  fats, 


90  FOUNDATIONS   OF   BIOLOGY 

and  proteins  must  be  oxidized,  that  is,  burned,  and  conse- 
quently free  oxygen  transmitted  throughout  the  plant  to  the 
various  cells,  and  carbon  dioxide  carried  away.  This  is 
effected  by  an  intercellular  system  of  air  spaces  which  rami- 
fies throughout  the  plant  and  communicates  with  the  sur- 
rounding atmosphere  chiefly  by  way  of  the  stomata. 

We  have  now  considered,  in  such  detail  as  the  scope  of  the 
present  work  requires,  the  structure  and  functions  of  a  typical 
higher  plant  as  a  whole,  and  have  indicated  how  the  organ- 
ism is  specialized  for  the  chief  function  which  primarily 
concerns  the  individual;  that  is,  nutrition,  or  the  trans- 
formation of  matter  and  energy  into  life  and  work.  Since, 
however,  the  duration  of  the  existence  of  the  individual  is 
relatively  limited,  it  is  obvious  that  some  provision  must 
exist  for  the  continuation  of  the  race.  In  other  words  new 
individuals  must  be  formed.  This  brings  us  to  the  second 
great  function  of  the  organism,  reproduction. 


CHAPTER  IX 
REPRODUCTION  IN  PLANTS 

The  synthetic  act  by  which  the  organism  maintains  itself  is 
fundamentally  of  the  same  nature  as  that  by  which  it  repairs 
itself  when  it  has  undergone  mutilation,  and  by  which  it  multi- 
plies and  reproduces  itself. — Bernard 

AMONG  the  lowest  members  of  the  plant  kingdom  the  body 
consists  of  but  a  single  cell;  the  individual  and  the  cell  are 
identical.  As  has  been  seen  irr  Sphaerella,  all  the  life  pro- 
cesses essential  to  the  individual  are  exhibited  in  relative 
simplicity  and  without  obviously  complicated  machinery. 
Moreover,  the  continuation  of  the  race  is  provided  for  by  the 
individual  cell  dividing  to  form  two  new  cells.  Neglecting 
for  the  time  being  the  mechanism  of  cell  division,  it  is  clear 
that  reproduction  in  Sphaerella,  since  it  is  not  complicated  by 
specialized  organs  for  its  performance,  is  a  comparatively 
simple  process. 

We  have  considered  briefly  the  gradual  increase  in  com- 
plexity of  the  plant  body  from  the  unicellular  condition, 
through  colonies  of  essentially  similar  cells  and  the  thallus 
type,  to  that  of  the  higher  vascular  plants,  placing  emphasis 
on  organs  directly  or  indirectly  associated  with  nutrition. 
It  is  necessary  now  to  review  in  a  similar  manner  the  speciali- 
zations of  structure  and  function  which  exist  in  the  plant 
kingdom  for  the  multiplication  of  individuals. 

It  may  be  well  to  reiterate  here  that  reproduction  and 
growth  are  phenomena  which  are  intrinsically  the  same  — 
both  are  the  result  of  a  preponderance  of  the  constructive 

91 


92  FOUNDATIONS   OF   BIOLOGY 

phase  of  metabolism.  The  single  cell,  whether  a  whole 
organism  or  a  single  unit  of  a  complex  body,  increases  in 
volume  up  to  a  certain  limit  and  then  divides.  In  the  former 
case  two"Tie^wnMividuals  replace  the  parent  cell;  in  the 
latter,  liie^complelTbody  has  been  increased  to  the  extent  of 
one— celL-  In  both  "cases  cell  division  has  resulted  in  cell 
reproduction.  Thus  cell  division  is  always  reproduction, 
though  it  is  customary  and  convenient  to:restrict  the  term 
reproduction  to  cell  divisions  which  result  in  the  formation  of 
new  individuals — single  cells  or  groups  of  cells  which  sooner 
or  later  separate  from  the  parent  organism. 

It  will  be  recalled  that  during  the  life  cycle  of  Sphaerella 
there  is  associated  with  the  reproductive  act  of  cell  division, 
the  formation  of  cell  individuals  which  exhibit  in  simple  form 
the  fundamental  characteristics  of  spores  and  gametes.  We 
shall  now  see  that  the  development  and  specialization  of 
these  is  at  the  basis  of  the  elaborate  reproductive  processes  of 
the  higher  plants. 

A.   SPORE  FORMATION 

As  already  emphasized,  cell  division  among  unicellular 
plants  results  in  the  formation  of  new  individuals,  and,  among 
multicellular  plants,  in  the  growth  of  the  single  individual. 
This  is  well  illustrated  by  the  familiar  pond  scums  in  which 
the  plant  body  consists  of  a  series  of  similar  cells  placed  end 
to  end  to  form  a  long  thread-like  body.  In  such  cases,  cell 
division  results  merely  in  an  increase  in  the  length  of  the  fila- 
ment constituting  the  plant  body,  unless  the  newly  formed 
cell  becomes  detached  from  the  parent  plant.  As  a  matter 
of  fact,  however,  under  certain  conditions  the  protoplasmic 
content  actually  does  make  its  escape  from  the  cell  wall  and 
swims  about  in  the  surrounding  water.  This  independent 
PROTOPLAST  is  a  spore. 


REPRODUCTION   IN   PLANTS  93 

Moreover,  this  spore  now  begins  a  series  of  cell  divisions 
which  result  in  a  new  filamont,  or  individual.  It  will  be  noted 
that  the  potentialities  of  the  spore  and  the  protoplasts  which 
continue  to  retain  their  stations  in  the  parent  body  are  in- 
trinsically the  same,  but  the  opportunity  of  the  spore  is  dif- 
ferent. In  brief,  the  fact  that  the  spore  has  separated  from 
the  parent  stock  appears  to  be^the^r^ason^why  it  reproduces. 
Therefore  a  SPORE  may  be  defined  as  a  cell,  or  the  essential 
part  of  a  cell,  the  protoplast,  which  has  separated  from  one 
plant  body  and  is  capable  of  producing  another  plant  body. 
This  statement  might,  at  first  glance,  seem  to  indicate  that 
spore  formation  is  restricted  to  plants  with  multicellular 
bodies,  whereas  we  have  seenjthat  s^ore  formation  occurs 
in  the  life  cycle  of  Sphaerella.  This  apparent  contradiction  is 
cleared  away  when  we  recall  that  in  the  latter  the  cell  divi- 
sions which  produce  the  spores  do  not  involve  the  cell  wall; 
merely  the  protoplast  within  divides  and  the  daughter  cells 
make  their  escape.  (Fig.  9.) 

Therefore  spore  formation  is  not  a  necessary  result  of  the 
establishment  of  a  multicellular  body,  but  an  inheritance 
from  unicellular  forms  which  makes  possible  one  of  the  two 
effective  types  of  asexual  reproduction  in  the  Metaphyta. 
The  other  type  is  FRAGMENTATION,  which  consists  essentially 
in  the  separation  from  the  body  of  larger  or  smaller  parts, 
which  later  reproduce  the  whole  plant.  It  is  a  familiar  fact 
that,  under  proper  conditions,  cuttings,  buds,  bulbs,  and 
sometimes  pieces  of  leaves  may  reproduce  or,  as  it  is  some- 
times stated,  REGENERATE  a  complete  plant.  This  is  just  an 
expression  of  the  same  power  which  the  spore,  though  a  single 
cell,  exhibits.  It  regenerates,  as  it  were,  a  plant  body  similar 
to  the  one  from  which  it  has  separated. 


94  FOUNDATIONS   OF   BIOLOGY 


B.  GAMETE  FORMATION 

In  the  life  cycle  of  Sphaerella  it  was  noted  that  under  cer- 
tain conditions  the  so-called  dormant  cell,  instead  of  dividing 
twice  to  form  four  spores,  divides  five  or  six  times  and  forms 
32  to  64  small  cells  called  gametes.  Now  it  is  not  the  struc- 
ture but  the  behavior  of  the  gametes  which  particularly  dis- 
tinguishes them  from  spores.  From  the  standpoint  of  their 
origin,  gametes  may  be  regarded  as  spores  which  have  de- 
veloped the  habit  of  fusing  to  form  a  zygote.  Moreover, 
the  origin  of  gametes  is  the  origin  of  SEX,  so  that  sexuality 
arose  in  plants  when  spores,  instead  of  reproducing,  devel- 
oped the  habit  of  pairing  and  thus  became  gametes.  The 
act  of  fusing  is  FERTILIZATION  and  the  cells  which  unite  are 
sex  cells. 

A  concrete  example  may  emphasize  this  important  point. 
The  body  of  a  filamentous  Alga,  Ulothrix,  is  composed  of 
a  linear  series  of  cells  all  of  which  are  essentially  the  same  in 
structure  and  function.  Under  favorable  conditions  the 
cells  divide  and  the  plant  grows  in  length.  New  individuals 
are  not  formed  by  this  process,  although  the  mechanical 
breaking  of  the  filament  into  two  parts,  owing  to  the  sim- 
plicity of  the  body,  gives  two  individuals.  When  conditions 
become  less  favorable  for  vegetative  growth,  some  of  the 
cells  cease  to  contribute  to  the  elongation  of  the  filament. 
Instead,  the  protoplasts  begin  to  divide  within  their  cell 
walls,  and  thus  each  forms  from  2  to  64  or  more  spores  of 
different  sizes,  depending  upon  the  number  of  divisions  the 
parent  protoplast  undergoes.  (Fig.  49.) 

The  largest  spores  are  provided  witfci  four,  and  the  smallest 
with  two,  flagella  by  means  of  which  they  swim  actively  in 
the  water  when  discharged  from  the  parent  plant  body. 
However,  the  number  of  flagella  is  apparently  of  no  im- 


REPRODUCTION    IN    PLANTS 


95 


portance  since  the  cells  of  intermediate  size  may  have  either 
two  or  four.  Nevertheless  the  behavior  of  the  spores  of  dif- 
ferent sizes  is  characteristic  and  significant.  The  largest 
spores  spon  settle  down  and,  attaching  themselves  by  the 
flagellated  end,  begin  to  develop  into  new  filaments.  The 
spores  intermediate  in  size  likewise  form  new  individuals, 


\\ 


FIG.  49.  —  Vlothrix,  a  filamentous  Green  Alga.  A,  modified  cell  for  attach- 
ment at  the  base  of  a  filament.  B,  cells  of  a  filament  which  have  formed 
spores.  From  three  cells  the  spores  have  been  liberated.  C,  part  of  a  filament 
liberating  spores  (below),  and  gametes  (above)  which  pair  to  form  zygotes. 
(From  Coulter.) 

but  the  process  is  much  less  rapid;  while  the  smallest  spores 
not  only  germinate  very  slowly,  but  give  rise  to  dwarf  fila- 
ments with  vigor  below  the  normal.  As  a  matter  of  fact  very 
few  of  the  smallest  spores  germinate  at  all.  Instead,  they 
unite  in  pairs,  each  pair  fusing  to  form  a  large  single  cell.  It 
is  apparent  that  the  small  spores  by  fusing,  instead  of  feebly 
germinating,  perform  the  sex  act  and,  therefore,  are  gametes, 
while  the  product  of  this  process  of  fertilization  is  a  zygote. 


96  FOUNDATIONS   OF   BIOLOGY 

Nothing  could  indicate  more  clearly  the  primary  relationship 
of  gametes  to  spores  than  the  origin  of  sex  and  sexual  repro- 
duction through  the  assumption  by  certain  spores  of  the  habit 
of  pairing  to  form  a  zygote  before  germination. 

It  should  be  noted  that  sexual  reproduction  is  not  a  differ- 
ent kind  of  reproduction,  but  merely  reproduction  preceded 
by  the  formation  of  a  zygote;  a  fact  very  readily  lost  sight 
of  in  the  higher  forms  where  accessory  phenomena  connected 
with  sexuality  obscure  the  essential  features,  but  quite  ap- 
parent in  Ulothrix  because  here  the  zygote  does  not  form 
directly  a  new  filament.  Instead,  after  passing  a  longer  or 
shorter  time  in  a  dormant  condition  protected  by  a  heavy 
wall,  the  protoplast  (zygote)  within  divides  to  form  a  number 
of  spores,  each  of  which  then  germinates  into  a  new  indi- 
vidual. Thus  in  Ulothrix,  as  in  Sphaerella,  reproduction  is 
solely  by  spores,  "sexual  'reproduction'  not  reproducing,  but 
only  protecting  a  spore-forming  protoplast." 

C.  SEX  DIFFERENTIATION 

So  far  we  have  seen  that  sex  cells,  the  gametes,  arose  with 
the  establishment  of  the  habit  of  reduced  spores  uniting  in 
pairs.  This  is  obviously  a  statement  of  fact  rather  than  an 
explanation  of  sex.  Although  the  two  cells  which  fuse  show 
no  morphological  characters  by  which  they  can  be  distin- 
guished from  each  other,  there  is  certainly  a  physiological 
basis  of  sex  which  induces  them  to  swim  toward  each  other, 
to  become  oriented  so  that  fusion  begins  at  the  flagellated 
ends,  and  to  melt  into  a  single  cell,  which  culminates  in  a 
reorganized  cell  with  the  complicated  structural  and  physio- 
logical equipment  of  the  two  cells  which  entered  into  its 
make-up.  The  zygote  thus  is  a  cell  which  combines  the 
characteristics  of  both  the  contributing  gametes,  and  to  this 


REPRODUCTION   IN   PLANTS 


97 


significant  fact  must  be  attributed  the  profound  importance 
of  sex  phenomena  in  the  life  history  of  plants  as  well  as  of 
animals. 

Although  sexuality  is  fundamentally  a  physiological  dif- 
ference between  gametes  which 
leads  to  their  characteristic 
behavior  (zygote  formation) , 
even  among  the  lower  plants 
structural  differentiations  ap- 
pear. In  fact,  a  series  of 
plants  can  be  arranged  show- 
ing a  gradual  transition  from 
gametes  which  are  morpholog- 
ically identical  to  those  which 
differ  so  widely  that  they  ap- 
pear to  have  little  in  com- 
mon. Oedogonium,  an  un- 
branched  filamentous  Alga, 
will  suffice  as  an  example, 
since  it  affords  an  excellent 
illustration  of  an  intermediate 
stage  in  gamete  differentiation. 
One  form  of  Oedogonium 
gamete,  representing  an  entire 
protoplast  of  a  greatly  enlarged 
cell,  is  richly  supplied  with 
food  materials  and  chloroplas- 
tids  and  remains  motionless 
within  the  cell  wall.  The 

other  type  develops  in  pairs  in  small  cells  with  greatly  re- 
duced chloroplastids  and  food  content.  Instead  of  being 
motionless,  each  cell  is  provided  with  a  circlet  of  cilia  by 
which  it  leaves  its  place  of  origin,  swims  actively  in  the 


FIG.  50.  —  Oedogonium,  a  filamentous 
Green  Alga.  A,  young  filament.  B,  por- 
tion of  a  filament  forming  gametes  (egg 
and  sperm) .  Below  are  two  sperm  which 
have  just  been  liberated;  above  is  a  large 
egg  with  a  sperm  just  coming  into  con- 
tact with  it  to  form  a  zygote.  (From 
Coulter.) 


FOUNDATIONS   OF  BIOLOGY 


water  and,  entering  a  cleft  in  the  wall  surrounding  a  large 
gamete,  fuses  with  it  to  form  a  zygote.     (Figs.  50,  51.) 

In  short,  one  gamete,  designated  the  EGG,  is  a  large  non- 
motile  cell  stored  with  food  materials,  while  the  other 
gamete,  or  SPERM,  is  a  small  active  cell  largely  devoid  of 
food.  This  is  typical  of  the  conditions  which  are  at  the  foun- 
dation of  gamete  differentiation  throughout  the  plant  and 
animal  kingdoms  —  eggs  and  sperm  expressing  a  physiologi- 
cal division  of 
labor  which  en- 
tails structural 
specialization 
in  opposite  di- 
rections. 

In  Oedogo- 
nium  sexuality 
is  apparent  both 
in  the  behavior 
and  in  the 
structure  of  the 
gametes,  so 
that  it  is  pos- 
sible to  identify 
the  sex  cells  as  MALE  gametes,  or  sperm,  and  FEMALE  gametes, 
or  eggs.  It  will  be  noted  that  this  is  not  the  origin  of  sex, 
for  sex  arose  when  spores  by  their  behavior  became  gametes. 
In  other  words,  the  sex  act  is  the  fusion  of  two  cells  which 
reorganize  as  a  single  cell;  and  all  modifications  of  these  cells, 
which  enable  them  to  function  as  gametes,  are  secondary. 

D.   REPRODUCTIVE  ORGANS 

Hand  in  hand  with  the  specialization  of  spores  and  gametes 
there  is  a  progressive  modification  of  the  cells  or  groups  of 


FIG.  51.  —  Oedogonium;  A,  zygote  emerging  from  cell  of 
parent  filament.  B,  division  of  zygote  into  four  spores.  C, 
mature  spores  ready  to  escape  and  develop  into  new  fila- 
ments. Note  that  the  zygote  does  not  directly  give  rise  to 
a  filament,  but  to  spores.  (From  Coulter.) 


REPRODUCTION   IN   PLANTS 


99 


cells  which  produce  them,  until  highly  developed  REPRODUC- 
TIVE ORGANS  arise.  The  asexual  reproductive  cells  are  formed 
in  SPORANGIA,  which  may  be  merely  vegetative  cells  in  which 
the  protoplast  becomes  transformed  into  a  spore,  or  elaborate 
multicellular  structures  set  aside  for  this  one  function.  Simi- 
larly, with  the  origin  of  sexuality,  the  sex  cells  arise  in  GAME- 
TANGIA,  which  later  are 
distinguished  as  ANTHE- 
RIDIA,  or  sperm-produc- 
ing, and  ARCHEGONIA,  or 
egg-producing  organs. 
Moreover,  although  the 
terms  male  and  female 
are  strictly  applicable 
only  to  the  sperm  and 
eggs  respectively,  the  an- 
theridia  and  archegonia 
are  called  male  and  fe- 
male organs;  while  a 
plant  body  which  bears 
only  male  reproductive 
organs  is  designated  as  a 
male  plant  and  one  which 
bears  female  reproduc- 
tive organs  is  known  as 
a  female  plant.  In  short,  the  sexuality  of  the  gametes  is 
reflected  back,  as  it  were,  to  the  organs  and  then  to  the 
individual  which  bears  them;  although  actually  the  gametes 
are  the  only  sex  cells.  If  this  is  kept  clearly  in  mind  it  will 
obviate  confusion  in  considering  the  remarkably  specialized 
secondary  features  which  sexuality  imposes  on  the  bodies  of 
higher  plants  and  animals.  (Fig.  52.) 

We  may  now  recapitulate  before  proceeding  to  further 


FIG.  52. — A  Brown  Alga,  Ectocarpus.  A, 
portion  of  a  filament  with  a  sporangium  and  a 
liberated  spore;  B,  portion  of  a  filament  with 
a  gametangium  and  a  liberated  gamete.  (From 
Coulter.) 


100  FOUNDATIONS   OF   BIOLOGY 

complications.  Beproduction,  divested  of  its  specialized 
features,  is  merely  growth  expressed  in  cell  divisions.  This 
primary  potentiality  of  all  cells  may  exist  side  by  side 
with  the  development  of  cells  specialized  for  asexual  repro- 
duction (spores)  and  sexual  reproduction  (gametes).  In 
either  case  the  products  become  separated  from  the  parent 
body  and  develop  new  bodies.  Furthermore,  spores  which 
at  first  are  developed  from  any  of  the  vegetative  cells  of  the 
plant  body,  later  arise  in  asexual  reproductive  organs 
(sporangia),  while  gametes  are  produced  in  sexual  reproduc- 
tive organs  (gametangia) .  With  the  morphological  differen- 
tiation of  gametes  into  sperm  and  eggs,  a  further  specializa- 
tion of  the  gamete-forming  organs  results  in  male  and  female 
reproductive  organs  (antheridia  and  archegonia).  When 
sporangia  and  gametangia  are  borne  by  separate  individuals, 
asexual  plants  (SPOROPHYTES)  and  sexual  plants  (GAMETO- 
PHYTES)  result.  Finally,  the  sperm  and  eggs  may  be  borne 
on  separate  gametophytes,  in  which  case  male  and  female 
gametophytes  result. 

E.   ALTERNATION  OF  GENERATIONS 

From  the  standpoint  of  the  evolution  of  the  higher  plants 
the  most  significant  fact  stated  above  is  that  sporangia  and 
gametangia  may  be  borne  by  separate  individuals,  for  this 
clearly  involves  an  asexual,  spore-bearing  generation,  and  a 
sexual,  gamete-bearing  generation.  We  shall  outline  this 
alternation  of  generations  in  the  life  history  of  a  typical  Moss 
and  Fern  as  an  introduction  to  the  problem  of  reproduction 
in  the  higher  Flowering  Plants. 

1.   The  Moss 

The  common  Mosses  of  woods,  hillsides,  and  fields  are  a 
relatively  inconspicuous  but  nevertheless  an  important  part 


REPRODUCTION   IN   PLANTfc 


101 


of  our  flora,  since  they  form  heavy  growths  or  carpets  of  vege- 
tation which  hold  back  much  of  the  rainfall  so  that  it  sinks 
into  the  soil.  Although  there  are  over  8000  species  which 
botanists  include  in  the  order  Bryales  of  the  Phylum  BRYO- 


Fia.  53.  —  The  life  history  of  a  Moss,  chiefly  Polytrichum.  a,  the  entire  plant 
(gametophyte  and  sporophyte),  XI.  b,  median  vertical  section  of  the  capsule  in 
which  spores  are  formed,  X  6,  with  spore  (c)  and  germinating  spore  (d),  X  300;  e, 
spore  germinated  to  a  protonema  with  a  bud  which  forms  leafy  plant  (gametophyte), 
X  75;  /,  tip  of  gametophyte  with  two  archegonia,  X  2;  g,  archegonium  in  section 
showing  egg,  X  16;  h,  tip  of  gametophyte  with  antheridia,  X  2;  i,  antheridium,  X  16; 
j,  a  single  liberated  sperm,  X  600;  k,  gametophyte  with  sporophyte  developing  in 
enlarged  and  transformed  archegonium.  (After  Ganong,  Dodel-Port  and  others.) 

PHYTA,  a  general  description  of  a  typical  common  Moss,  such 
as  Polytrichum  communae,  will  suffice  for  the  purpose  at 
hand.     (Fig.  53.) 
The  shoot  of  a  moss  plant  is  differentiated  into  stem  and 


102  FOtJNDATiONS   OF    BIOLOGY 

leaves  which  are  of  very  simple  construction  compared  with 
those  of  the  Flowering  Plant  we  have  studied.  True  roots  are 
not  present,  but  their  function  is  performed  by  filamentous 
outgrowths  called  RHIZOIDS.  At  the  top  of  the  leafy  moss 
plant,  inconspicuous  reproductive  organs  are  developed. 
Some  species  bear  both  antheridia  and  archegonia  on  the 
same  plant,  while  others  have  only  one  type.  The  leafy  moss 
plant  is  thus  a  sexual  individual,  or  gametophyte.  When  the 
reproductive  organs  are  mature,  sperm  escape  from  the 
antheridia  and,  swimming  about  in  moisture  which  has  col- 
lected on  the  leaves,  are  attracted  to  the  archegonia  contain- 
ing the  eggs,  apparently  by  a  chemical  substance  secreted 
within  these  organs.  A  single  sperm  which  has  made  its  way 
down  into  an  archegonium,  fuses  with  the  egg  to  form  a 
zygote.  The  fertilized  egg  retains  its  position  in  the  arche- 
gonium and  germinates.  The  result  is  a  rod-shaped  embryo 
which  grows  not  only  upward  through  the  archegonium  and 
so  out  into  the  world,  but  also  downward  into  the  tissues  of 
the  gametophyte,  from  which  it  secures  practically  all  of  its 
food  materials. 

The  essentially  parasitic  nature  of  the  new  individual 
renders  the  development  of  leaves  superfluous,  so  it  consists 
of  a  simple  upright  stalk  at  the  top  of  which  reproductive 
organs  are  borne.  These  are  sporangia  and  accordingly  the 
individual  is  a  sporophyte.  The  ripe  spores  are  liberated  and, 
falling  to  the  ground,  each  forms  a  filamentous  outgrowth 
called  a  PROTONEMA.  Soon  a  bud  arises  on  the  protonema 
which  develops  into  a  leafy  moss  plant. 

A  common  Moss  thus  exhibits  in  its  life  history  an  alterna- 
tion of  sexual  and  non-sexual  generations.  The  leafy  moss 
plant,  with  antheridia  and  archegonia,  produces  gametes 
and  is  the  gametophyte.  The  leafless  generation,  which 
develops  from  the  fertilized  egg  in  the  archegonium,  produces 


REPRODUCTION   IN    PLANTS  103 

spores  and  is  the  sporophyte.  The  gametophyte  arises 
asexually,  but  is  itself  sexual;  the  sporophyte  arises  sexually 
but  is  itself  asexual.  The  dominant  generation  from  the 
viewpoint  of  both  structure  and  nutrition  —  the  plant  one 
thinks  of  as  a  'moss'  —  is  the  gametophyte. 

2.   The  Fern 

The  common  Ferns  comprise  the  largest  group  of  one  of  the 
major  divisions  of  the  plant  kingdom  known  as  the  PTERIDO- 
PHYTA.  Although  the  forms  of  different  species  are  remark- 
ably varied,  the  ensemble  of  characters  and  in  particular 
the  foliage  is  quite  distinctive,  so  that  one  would  recognize 
practically  any  member  of  the  group  as  a  'fern.'  The  stems 
may  be  short  and  close  to  the  ground,  or  upright  as  in  the 
Tree  Ferns,  though  creeping  and  underground  stems 
(rhizomes)  are  more  common.  The  leaves,  known  as  FRONDS, 
either  arise  in  clusters  from  the  tip  of  the  stem  (Tree  Ferns), 
or  are  distributed  along  the  creeping  and  underground  stems. 
Roots  bring  the  stem  into  intimate  contact  with  the  food 
materials  of  the  soil,  though  rhizomes  function  to  a  certain 
extent  as  roots.  An  examination  of  the  cellular  structure  of 
a  common  Fern,  such  as  Aspidium  marginale,  shows  that  it 
is  much  more  complex  than  a  Moss,  the  tissues  of  stem  and 
leaves  being  essentially  like  those  we  have  seen  in  the  Flower- 
ing Plants,  and  accordingly  Ferns  and  Flowering  Plants  are 
frequently  referred  to  as  vascular  plants. 

The  leafy  fern  plant  bears,  on  certain  of  its  fronds,  repro- 
ductive organs  which  are  sporangia.  These,  of  course,  pro- 
duce spores  and  therefore  the  plant  commonly  recognized  as 
a  Fern  is  a  sporophyte.  The  spores  when  ripe  are  liberated 
from  the  sporangia  and  fall  to  the  ground,  where  they  germi- 
nate. From  the  spore  arises  a  tiny  body,  about  a  quarter  of 
an  inch  in  diameter,  called  a  PROTHALLUS,  which  is  essentially 


104 


FOUNDATIONS   OF   BIOLOGY 


a  plate  of  chlorophyll-bearing  cells  with  rhizoids  attaching 
it  to  the  ground.  On  its  lower  surface  are  developed  repro- 
ductive organs,  antheridia  and  archegonia,  which  form  gam- 
etes. The  prothallus  therefore  is  a  gametophyte.  (Fig.  54.) 

Sperm  are  liberated  from  the  antheridia  and  swim  in  the 
moisture  from  dew  or  rain  to  the  archegonia.    A  single  sperm 


FIG.  o4.  —  The  life  history  of  a  common  Fern,  chiefly  Aspidium.  a,  the  entire 
sporophyte,  X  A;  b,  portion  of  a  leaf  showing  groups  of  sporangia  (sori),  X  3;  c,  a 
sorus  showing  sporangia,  X  10;  d,  a  sporangium,  X  50;  e,  a  single  spore,  X  100; 
/,  ventral  view,  X  3  and  g,  a  median  section,  X  6  of  a  prothallus  showing  rhizoids,  anthe- 
ridia, and  archegonia;  h,  antheridium  liberating  sperm,  X  120;  i,  single  sperm  still  at- 
tached to  a  remnant  of  'mother  cell',  X  300;  j,  open  archegonium  with  sperm  passing 
down  to  egg,  X  120;  k,  young  sporophyte  developing  from  zygote.  (After  GanongJ 

works  its  way  down  an  archegonium  and  fuses  with  the  egg 
to  form  a  zygote.  Then  the  zygote,  which  remains  in  the 
archegonium,  proceeds  to  divide  and  forms  at  first  a  small 
plant,  with  stem  and  leaf  which  grows  upward  and  root 
which  seeks  the  soil.  During  the  process  of  root  and  shoot 
development  the  plant  retains  its  attachment  to  the  parent 


REPRODUCTION   IN   PLANTS 


105 


prothallus  from  which  its  food  is  secured.  Later,  when  direct 
communication  with  the  environment  has  been  established 
by  its  own  root  and  leaf,  the  new  generation  becomes  entirely 
independent  of  the  prothallus,  which  then  degenerates  and 
dies.  The  young  plant  gradually  grows  into  the  typical 
asexual  leafy  fern  plant,  which  itself  in  due  time  produces 
spores. 

It  is  clear  that  in  the  Fern,  as  in  the  Moss,  there  is  an  al- 
ternation of  generations.  The  leafy  fern  plant  (sporophyte) 
gives  rise  to  the  prothallus  (gametophyte) .  The  leafy  fern 

Ferns          Flowering  Plants 


Fio.  55.  —  Diagram  to  illustrate  the  decline  of  the  gametophyte  generation 
and  the  advance  of  the  sporophyte  generation.     (From  Coulter.) 

arises  sexually,  but  is  itself  asexual;  the  prothallus  arises 
asexually,  but  is  itself  sexual.  The  significant  fact,  however, 
is  that  the  conspicuous  leafy  moss  plant  is  a  gametophyte, 
while  the  large  leafy  fern  plant  is  a  sporophyte;  or,  one 
may  say,  the  'moss'  is  a  sexual  plant  and  the  'fern'  is  an 
asexual  plant.  This  ascendancy  in  dominance  of  the  asexual 
and  suppression  of  the  sexual  generation,  which  is  so  charac- 
teristic of  the  fern  as  compared  with  the  moss  life  history,  is 
carried  still  further  in  the  higher  Ferns  and  finally  culminates 
in  the  Flowering  Plants.  (Fig.  55.) 

3.   Higher  Ferns 

As  we  have  seen,  the  sporophyte  of  the  common  Ferns  pro- 
duces spores  on  ordinary  vegetative  fronds  or,  more  rarely,  on 
specialized  sporophylls.  (Figs.  39,  54.)  In  either  case  but  one 


106 


FOUNDATIONS   OF   BIOLOGY 


kind  of  spore  is  formed.  Among  the  higher  Ferns,  however, 
spores  of  two  kinds  occur  which,  since  they  differ  greatly  in 
size,  are  called  MICROSPORES  and  MEGASPORES.  The  produc- 
tion of  two  kinds  of  spores  is 
known  as  HETEROSPORY  and 
leads  to  the  differentiation  of 
the  sporophylls  into  MICRO- 

SPOROPHYLLS    and    MEGASPO- 

ROPHYLLS.  Moreover,  the 
microspores  on  germination 
form  gametophytes  which 
produce  sperm,  and  therefore 
are  called  MALE  GAMETO- 
PHYTES, while  the  megaspores 
develop  into  gametophytes 
bearing  eggs,  and  accordingly 
are  known  as  FEMALE  GAME- 
TOPHYTES. Finally,  in  these 
heterosporous  forms,  the 
gametophytes  are  no  longer 
even  small  independent 
plants,  such  as  the  prothallus 
of  the  common  Ferns,  but 
both  male  and  female  gameto- 
phytes are  so  greatly  reduced 
that  they  practically  remain 
permanently  in  the  parent 
microspore  and  megaspore, 
respectively,  which  supply 
them  with  food.  This,  it  will 
be  noted,  is  just  the  reverse  of  the  condition  which  exists  in 
the  Moss,  where  it  is  the  sporophyte  which  is  the  dependent 
generation.  (Figs.  56,  57.) 


FIG.  50.  —  Stages  in  the  life  history  of 
a  higher  Fern  (Marsilia).  A,  micro- 
spore,  enclosing  the  male  gametophyte 
with  two  groups  of  sperm  mother  cells, 
and  prothallial  cells  (p);  B,  sperm; 
C,  megaspore,  enclosing  food  material 
(starch  grains),  and  female  gametophyte 
comprising  a  single  archegonium  (with 
egg)  at  one  end  of  the  spore;  D,  a  week- 
old  embryo  sporophyte,  still  attached  to 
the  megaspore,  with  first  leaf  (I)  and 
root  (r).  (From  Bergen  and  Davis.) 


REPRODUCTION    IN    PLANTS 


107 


Fig.  57.  —  A  microspore  and 
megaspore  of  a  'higher  Fern', 
Selaginella,  magnified  and  drawn 
to  the  same  scale.  (From 
Coulter.) 


4.  Flowering  Plants 
Passing  to  the  Flowering  Plants, 
we  find  that  these  are  heterosporous 
sporophytes,  and  the  FLOWER  rep- 
resents a  greatly  modified  stem 
(branch),  the  leaves  of  which  are 
specialized  as  sporophylls  and  ac- 
cessory structures.  In  order  to 
make  this  clear  it  is  necessary  to 
review  the  structure  of  a  typical 
flower.  (Figs.  40,  58.) 

A  complete  flower  consists  of  four  whorls  of  modified 
leaves.  These  arise  near  together  at  the  tip  of  a  PEDUNCLE, 
representing  the  floral  branch,  which  connects  the  flower 
proper  with  the  main  tissue  systems  of  the  plant  as  a  whole. 
The  outer  and  lower  circle  of  leaves 
(CALYX)  is  composed  of  several 
parts  (SEPALS)  which  usually  are 
green  and  retain  a  leaf-like  appear- 
ance. Just  within  and  above  the 
calyx  is  the  second  circle  (COROLLA) 
formed  of  larger  leaves  (PETALS) 
which  are  usually  brightly  colored. 
The  calyx  and  corolla  together  form 
the  PERIANTH,  or  floral  envelope 
which  surrounds  the  primary  floral 
organs,  the  STAMENS  and  CARPELS. 

The  stamens  represent  the  third 
circle  of  leaves,  but  are  so  highly 
modified  that  their  leaf  origin  is  not 
—diately  apparent.  Each  con- 

6,  calyx;  c,  corolla;  d,  stamens;      gists    of    a   slender   FILAMENT   at   the 
e,  pistil  formed  of  fused  carpels.  ,.       .  .    .     .  , 

(Modified  from  Gager.)  apex  of  which  is  a  small  case  known 


108 


FOUNDATIONS   OF   BIOLOGY 


as  the  ANTHER.      Within  the  anther   POLLEN   GRAINS  are 
formed.    The  pollen  grains  are  microspores  and,  therefore,  it  is 


FIG.  59.  —  Transition  between  petals  and  stamens  in  a  Water  Lily.     (After  Gray.) 


apparent  that 


FIG.  60.  —  Dia- 
gram to  illustrate 
the  method  of  union 
of  three  carpels 
( megasporophylls ) 
to  form  the  ovule 
case  of  a  pistil 
(compound) .  The 
edges  which  unite 
form  the  point  of  at- 
tachment of  the  ov- 
ales.  (After  Gray.) 


the  pollen  sacs  of  the  anthers  are  MICROSPORAN- 
GIA  and  the  stamens  are  microsporophylls. 

Finally,  just  within  the  circle  of  stamens 
is  the  fourth  whorl  of  floral  leaves,  the  car- 
pels, in  which  specialization  has  gone  so  far 
that  practically  no  suggestion  of  leaf  struc- 
ture remains.  Each  carpel  consists  of  three 
parts:  a  lower,  expanded  portion  termed 
the  OVULE  CASE,  merging  above  into  the 
elongated,  slender  STYLE,  the  tip  of  which  is 
the  STIGMA.  Such  a  fully  developed  carpel 
is  known  as  a  PISTIL  and  when,  as  frequently 
happens,  the  various  carpels  fuse  to  form  a 
composite  structure,  this  is  termed  a  com- 
pound pistil.  Within  the  ovule  case  are  de- 
veloped the  reproductive  bodies  known  as  OV- 
ULES which  are  essentially  MEGASPORANGIA, 
for  within  each  is  formed  a  megaspore,  com- 
monly known  as  an  EMBRYO  SAC.  A  carpel, 
therefore,  is  a  megasporophyll.  (Fig.  60.) 


REPRODUCTION    IN    PLANTS  109 

So  far  it  is  clear  that  a  flower  is  a  group  of  sporophylls 
which  produce  microspores  and  megaspores.  Since,  how- 
ever, such  reproductive  bodies  always  form  male  and  female 
gametophytes,  their  development  must  now  be  considered. 

The  first  fact  to  have  clearly  in  mind  is  that  the  megaspore 
is  never  liberated  by  the  megasporangium.  And  further  that 
the  latter  remains  just  where  it  arose  in  the  ovule  case  of  the 
pistil.  Consequently  the  megaspore  germinates  within  the 
pistil,  and  it  forms  there  a  female  gametophyte  composed  of 
only  a  few  cells,  including  the  female  gamete,  or  egg.  Thus 
the  female  gametophyte  generation  of  Flowering  Plants  is  in- 
visible except  with  the  microscope. 

The  pollen  grain  is  a  typical  microspore,  a  single  cell  en- 
closed within  a  protective  wall.  Germination  starts,  while 
the  pollen  is  still  in  the  anther,  by  the  division  of  the  spore 
nucleus  into  two,  one  of  which  divides  again.  Further  devel- 
opment does  not  occur  unless  the  pollen  is  transferred  in 
some  way,  usually  by  insects  or  the  wind,  to  the  stigma  of 
the  pistil.  The  stigma  secretes  fluids  suitable  for  the  germina- 
tion of  the  ripe  pollen  grain,  which  bursts  its  rigid  wall  and 
puts  forth  a  cytoplasmic  tube.  This  grows  down  through  the 
tissues  of  the  pistil  until  its  tip  enters  the  ovule  case,  and 
carries  with  it  the  nuclei,  two  of  which  represent  sperm.  The 
pollen  has  now  completed  its  development  and  thus  the  con- 
tents of  the  pollen  grain  plus  the  tube  itself  constitute  a 
greatly  reduced  male  gametophyte. 

By  the  time  the  pollen  tube  reaches  the  ovule  case,  the 
megaspore  within  has  formed,  as  already  described,  the 
female  gametophyte  with  its  egg.  One  of  the  sperm  nuclei 
unites  with  the  egg  and  forms  a  zygote,  which  remains  just 
where  it  is,  surrounded  by  the  tissues  of  the  pistil  base,  and 
proceeds  to  divide  to  form  an  embryo  sporophyte  with  rudi- 
mentary root,  stem,  and  leaf.  Concurrently,  the  ovule  case 


110 


FOUNDATIONS   OF   BIOLOGY 


and  associated  tissues  of  the  base  of  the  pistil  undergo  more 
or  less  profound  changes  ('ripen')  and  become  transformed 
into  a  FRUIT.  The  young  sporophyte  within,  together  with 
food  material  for  its  further  development,  is  hermetically 
sealed  up  in  a  special  packet  —  it  has  become  a  SEED. 


FIG.  61.  —  The  life  history  of  a  higher  Flowering  Plant,  from  various  species,  a, 
shoot  of  a  Flax  with  flowers,  X  J;  b,  vertical  section  of  a  flower,  XI;  c,  an  anther 
cut  to  show  four  microsporangia  containing  pollen  grains  (microspores),  X6;  d,  an 
ungerminated  pollen  grain,  and  one,  e,  which  has  formed  tube  (male  gametophy te) , 
X  110;  /,  longitudinal  sections  of  an  ovule  enclosing  megaspore  and  its  contents 
(female  gametophyte) ,  X  20;  g,  an  ovule  transformed  into  a  seed  with  young  embryo 
sporophyte  and  endosperm,  X  10;  h,  a  mature  seed,  X  5;  i,  youjig  sporophyte  from 
the  germination  of  the  seed,  X  5.  (After  Ganong.) 

In  this  form  the  new  generation  is  prepared  not  only  to 
leave  the  parent  plant  and  withstand  adverse  conditions  for 
a  long  time,  but  also  to  continue  rapidly  its  development  into 
an  adult  sporophyte  when  it  falls  upon  favorable  soil.  Inci- 
dentally, it  may  be  mentioned  that  the  establishment  of  seed 
formation  is  probably  chiefly  responsible  for  the  dominant 
position  which  the  Flowering  Plants  hold  in  the  flora  of  to-day. 
(Fig.  61.) 


REPRODUCTION    IN    PLANTS  111 

Thus  it  is  clear  that  the  gametophyte  generation  of  Flower- 
ing Plants  is  reduced  to  practically  its  lowest  terms  —  a  few 
nuclear  divisions  sufficient  to  form  the  gametes.  The  whole 
generation  is  telescoped,  as  it  were,  within  the  flower  of  the 
previous  sporophyte  generation,  so  that  sporophyte  seems 
to  produce  sporophyte;  whereas,  as  a  matter  of  fact,  three 
distinct  generations  contribute  directly  to  the  formation  of 
the  seed.  A  seed  is  really  a  highly  modified  megasporangium 
with  its  contents.  The  seed  coat  comprises  tissue  from  the 
megasporangium  of  the  parent  sporophyte  bearing  the  flower 
(first  generation).  Certain  nutritive  tissues  (endosperm) 
represent  the  female  gametophyte  (second  generation).  The 
product  of  the  fertilized  egg  is 
a  young  sporophyte  (third 
generation).  (Fig.  62.) 

The  great  reduction  of  the 
gametophyte  generation  in 
Flowering  Plants  is  accom- 
panied by  a  transference  of 
some  of  the  phenomena  associ- 

ated    With     Sexuality     tO     the         FIG.  62.  —  Seed  of  a  Violet.  At  the  left 

sporophyte,  so  that  the  latter, 


though      intrinsically     asexual,      gametophyte)  enclosing  the  embryo,  or 

i  M  •        young  sporophyte.     (From  Coulter.) 

comes  secondarily  to  exhibit 

certain  sexual  characters,  chiefly  in  the  flower.  Thus, 
although  the  stamens  and  pistil  (carpels)  are  actually 
sporophylls  of  the  non-sexual  generation,  they  are  frequently 
referred  to  as  the  male  and  female  organs  of  the  flower. 
Likewise  POLLINATION,  or  the  transference  of  the  pollen 
grains  from  anther  to  stigma,  is  often  called  the  fertiliza- 
tion of  the  flower;  whereas,  as  we  have  seen,  it  is  merely  a 
preliminary  step  which  makes  it  possible  for  gametophytes 
to  meet  on  common  ground  so  that  the  sperm,  which  them- 


SPERM  ATOPHTTES       PTER1DOPHTTBS          BRYOPHYTES 


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112 


REPRODUCTION    IN    PLANTS  113 

selves  have  suffered  reduction  and  lost  their  motility,  can 
reach  the  egg  and  perform  the  act  of  fertilization.  Frequently 
this  sexual  differentiation  extends  to  the  flower  as  a  whole, 
since  some  flowers  bear  only  stamens  and  others  only  carpels 
and  are  known  as  male  and  as  female  flowers  respectively. 
Moreover,  male  and  female  flowers  may  be  borne  on  separate 
plants,  in  which  case  the  plants  themselves  are  called  male 
and  female.  In  brief,  the  terminology  which  rightly  is  appli- 
cable only  to  the  sexual  generation  is,  for  convenience,  trans- 
ferred to  the  asexual  generation,  in  consequence  of  the  fact 
that  vicarious  sexual  functions  are  reflected  back  to  it  through 
the  almost  complete  suppression  of  the  actual  sexual  genera- 
tion itself. 

If  we  glance  back  over  the  reproductive  processes  in  plants, 
we  are  impressed  with  the  fact  that  reproduction  is,  to  a 
very  large  extent,  asexual.  The  great  masses  of  thallus 
vegetation,  represented  by  Seaweeds  and  their  allies,  in- 
crease in  bulk  chiefly  by  vegetative  cell  division,  and  new 
individuals  are  formed  in  profusion  by  fragmentation  and 
spore  formation.  In  the  Mosses,  where  the  sexual  generation 
is  prominent,  beds  of  moss  are  developed  chiefly  by  the 
asexual  budding  of  the  sexual  plant,  while  spore  formation 
holds  a  prominent  place  in  the  increase  and  dissemination  of 
individuals.  In  the  Ferns  and  Flowering  Plants,  which  are 
to  all  intents  and  purposes  asexual  plants,  since  the  sexual 
phase  is  relegated  to  an  increasingly  obscure  position  in  the 
life  history,  reproduction  is  not  only  by  spores,  but  also  by 
cuttings,  bulbs,  fragments  of  leaves,  etc.  In  brief,  reproduc- 
tion, unaccompanied  by  sexual  phenomena,  is  apparently 
amply  sufficient  for  the  propagation  of  plants. 

However,  it  will  be  noted  that  sexuality  has  persisted  from 
its  simple  origin  when  spores  of  unicellular  plants  performed 
the  sexual  act  and  became  gametes.  The  gametophyte 


114  FOUNDATIONS   OF   BIOLOGY 

generation  in  the  life  history  which  it  provoked  wanes  in  im- 
portance as  we  proceed  from  the  lower  to  the  highest  plants, 
but  in  spite  of  this  the  sex  act  itself  is  retained  and  shows  its 
modifying  powers  even  in  the  asexual  generation.  Obviously 
some  advantages  must  be  gained  in  the  long  run  by  fertiliza- 
tion, other  than  the  establishment  of  another  generation 
in  the  life  history,  or  such  devious  methods  culminating 
in  the  flower  would  not  be  elaborated  for  its  preservation. 
We  shall  leave  this  large  problem  until  we  take  up  the  ques- 
tion of  sex  in  animals,  since  sexuality  is  a  fundamental  attri- 
bute of  both  plants  and  animals  which  profoundly  affects 
their  morphology  and  physiology. 


CHAPTER  X 
THE  ANIMAL  BODY 

If  we  contemplate  the  method  of  Nature,  we  see  that  every- 
where vast  results  are  brought  about  by  accumulating  minute 
actions.  — Spencer. 

THE  most  obvious  characteristic  which  distinguishes  fa- 
miliar plants  and  animals  is  the  power  of  locomotion  of  the 
latter.  This  criterion,  however,  fails  among  the  lowest  forms; 
for  example,  Sphaerella,  as  we  have  seen,  swims  as  actively  as 
Paramecium.  Moreover,  among  multicellular  animals  there 
are  innumerable  sessile  forms,  such  as  the  typical  stages  of 
the  Sponges,  Hydroids,  Barnacles,  etc.  Although  the  power 
of  locomotion  is  not  a  diagnostic  character  of  animals  as  com- 
pared with  plants  (this,  as  has  been  explained,  being  chiefly  a 
matter  of  metabolism),  it  is  a  fact  that,  taken  by  and  large, 
the  great  dissimilarity  between  the  bodies  of  multicellular 
plants  and  animals  is  a  direct  or  indirect  result  of  the  loss  by 
plants  and  the  development  by  animals  of  the  primitive  power 
of  locomotion  which  most  unicellular  organisms  possess.  At 
the  basis  of  this  difference  is  probably  the  fact  that  early  in 
the  evolution  of  plants  comparatively  rigid  cell  walls  of 
cellulose  were  established,  which  directed  the  development  of 
the  body  along  relatively  fixed  lines.  On  the  other  hand, 
animal  cells,  unhampered  by  the  limitations  imposed  by  rigid 
confining  walls,  were  free  to  respond  in  more  ways  to  environ- 
mental conditions,  and  this  made  possible  the  extremely 
diverse  forms  of  mobile  bodies  characteristic  of  the  animal 
kingdom.  This  greater  plasticity  of  the  animal  in  compari- 

115 


116  FOUNDATIONS   OF  BIOLOGY 

son  with  the  plant  is  reflected  again  in  the  fact  that  the  body 
of  the  higher  plants  is  essentially  a  combination  of  a  series 
of  tissue  systems  and  organs,  each  of  which  plays  a  particular 
part  in  the  economy  of  the  whole,  while  that  of  the  higher 
animal  is  a  cooperating  series  of  organs,  or  ORGAN  SYSTEMS. 
The  organ  systems  of  animals  may  be  classified  as  the 

INTEGUMENTARY  AND  SKELETAL  SYSTEMS  which  Constitute  the 

covering  and  the  framework  of  the  individual;  the  ALIMEN- 

TARY, RESPIRATORY,  CIRCULATORY,  and  EXCRETORY    SYSTEMS 

which  directly  or  indirectly  are  concerned  with  nutrition; 
the  NERVOUS  SYSTEM  which,  in  cooperation  with  the  system 
of  SENSE  ORGANS,  the  MUSCULAR  SYSTEM,  etc.,  not  only  coordi- 
nates the  various  parts  of  the  individual,  but  also  orients  the 
whole  with  respect  to  its  environment;  and,  finally,  the  RE- 
PRODUCTIVE SYSTEM  which  makes  possible  the  continuation 
of  the  race.  The  fundamental  life  processes  for  which 
these  systems  provide  must  be  carried  on  by  all  animals,  and 
the  chief  differences  in  the  structure  of  animals,  from  the 
lowest  to  the  highest,  is  a  resultant  of  the  means  adopted  to 
serve  these  essential  functions  under  different  exigencies  im- 
posed by  the  environment  and  mode  of  life. 

A.  THE  CHIEF  GROUPS  OF  ANIMALS 

The  animal  kingdom  may  be  divided  into  two  main 
groups  ;  on  the  one  hand,  thft  nnip.ft11nlfl.r  ariiTy^-Hnr  "PROTOZOA 
comprising  about  ten  thousand  known  kinds,  nearly  ^all 
of  which  are  microscopic,  such  as  Amopha, 


their  allies^  and  on  the  other  hand,  multicellular  forms,  or 
METAZOA.  The  latter  division  includes  animals  ranging  in 
size  from  those  which  are  so  small  that  hundreds  can  sport 
in  a  drop  of  water,  to  the  present-day  Whales  and  the  Dino- 
saurs of  the  past.  Although  the  actual  stages  in  the  transi- 
tion from  the  Protozoa  to  the  Metazoa  are  unknown,  among 


THE  ANIMAL  BODY  117 

the  more  complex  colonial  Protozoa  there  are  forms,  as  al- 
ready noted,  in  which  the  various  cells  become  organically 
connected  so  that  a  primitive  sort  of  body  results,  and,  fur- 
thermore, certain  cells  are  set  aside  for  reproduction.  In  other 
words,  cooperation  involving  a  physiological  division  of  labor 
takes  place  between  the  individuals  of  a  group  of  cells,  and 
this  results  in  the  establishment  of  an  individual  body  of 
somatic  cells  associated  with  germ  cells.  (Fig.  18.) 

The  Metazoa  proper  may  be  divided  into  two  large  groups 
known  as  INVERTEBRATES  and  VERTEBRATES.  The  former 
group,  frequently  referred  to  as  the  lower  animals,  comprises 
some  five  hundred  thousand  living  species  and  exhibits  an 
enormous  variety  of  form  and  complexity  of  structure  ranging 
from  the  Sponges  and  Hydroids  to  the  Molluscs,  Crustacea, 
and  Insects.  On  the  other  hand,  the  Vertebrates,  or  higher 
animals,  form  a  relatively  homogeneous  group  of  about  thirty- 
five  thousand  species,  including  the  FISHES,  AMPHIBIA,  REP- 
TILES, BIRDS,  and  MAMMALS.  The  Birds  and  Mammals  in 
contrast  with  all  other  animals  are  commonly  referred  to  as 
warm-blooded,  because  their  body  temperature  is  practically 
constant  and  usually  above  that  of  their  surroundings. 

The  highly  complicated  and  varied  organization  of  animals 
renders  it  impossible  to  present  a  concise  and  adequate  plan 
of  a  typical  animal  body,  and  it  is  therefore  necessary  in  the 
present  work  to  select  one  group  of  animals  as  the  basis  of 
study  and  then  to  compare  with  this,  in  so  far  as  comparisons 
are  possible  without  confusion,  a  few  of  the  most  significant 
morphological  and  physiological  variations  presented  by 
other  groups.  We  naturally  select  the  group  of  Vertebrates 
for  chief  consideration  not  only  because  its  relative  homo- 
geneity renders  it  the  most  available,  but  because  it  includes 
Man.  However,  even  before  we  focus  attention  on  the 
Vertebrates,  it  is  necessary  to  make  a  brief  preliminary  sur- 


118  FOUNDATIONS    OF   BIOLOGY 

vey  of  certain  morphological  principles  as  exhibited  among 
the  Invertebrates  —  selecting  as  types  the  Hydra,  Earth- 
worm, and  Crayfish  —  in  order  to  afford  a  background  for 
the  consideration  of  Vertebrate  structure  and  function. 

B.   HYDRA 

In  discussing  the  development  of  animals,  it  was  pointed 
out  that  the  dividing  egg  typically  forms  a  blastula  which,  in 
turn,  becomes  transformed  by  the  invagination  of  its  wall  at 
one  pole  into  the  gastrula  stage.  This  early  gastrula  is  essen- 
tially a  sac  composed  of  two  layers  of  cells :  an  outer  or  ecto- 
derm and  an  inner  or  endoderm  layer.  Although  no  adult 
animal  retains  this  simple  gastrula  form,  the  animals  com- 
posing the  group  known  as  the  COELENTERATES  are  to  all 
intents  and  purposes  permanent  gastrulae  since  their  bodies 
are  built  on  the  plan  of  a  two-layered  sac.  This  is  well  ex- 
hibited in  Hydra,  an  almost  microscopic,  fresh -water  Coelen- 
terate  which  is  commonly  found  attached  to  submerged 
vegetation  or  stones  in  brooks  and  ponds.  (See  p.  414.) 

The  body  of  Hydra  somewhat  resembles  a  tube  closed  at  one 
end,  constituting  the  FOOT,  and  open  at  the  other,  forming  the 
MOUTH.  Surrounding  the  mouth  is  a  circle  of  outpocketings 
of  the  body  wall  termed  TENTACLES.  The  main  axis  of  the 
body  extends  from  foot  to  mouth,  and  every  plane  passing 
through  this  axis  divides  the  body  into  symmetrical  halves. 
In  other  words,  the  parts  of  the  body  are  symmetrically  disr 
posed  about,  or  radiate  from,  the  main  axis,  and  so  Hydra 
affords  an  example  of  RADIAL  SYMMETRY.  (Fig.  64.) 

The  tubular  body  wall  of  Hydra  is  composed  of  two  dis- 
tinct cell  layers,  ectoderm  and  endoderm,  separated  by  a  thin 
non-cellular  supporting  layer  of  jelly-like  material  (MESO- 
GLOEA)  secreted  by  the  cells  of  both  ectoderm  and  endoderm. 
Hydra  thus  illustrates  a  simple  type  of  Metazoan  structure 


THE   ANIMAL   BODY 


119 


in  which  but  two  primitive  tissues  exist;  such  specializations 
as  are  necessary  for  the  performance  of  the  essential  life  func- 
tions being  confined  to  the  more  or  less  isolated  cells  of  these 
layers.  The  majority  of  the  cells  of  the  endoderm  which 


FIG.  64.  —  Hydra.  Longitudinal  section,  magnified.  1,  mouth;  2,  tentacles; 
3,  early  stage  in  budding;  4<  older  bud;  5,  ectoderm;  6,  endoderm;  7,  enteric 
cavity;  8,  testis;  9,  ovary.  (From  Linville  and  Kelly,  after  Parker.) 

line  the  ENTERIC  CAVITY  are  concerned  with  the  digestion  of 
solid  food  taken  in  through  the  mouth,  while  those  of  the 
ectoderm  are  variously  modified  for  protection,  and  the  other 
relations  of  the  individual  to  its  surroundings,  as  well  as  for 
reproduction. 

In  short,  in  the  organization  of  Hydra  the  primary  tissues 
(ectoderm  and  endoderm)  have  not  become  differentiated 


120 


FOUNDATIONS   OF   BIOLOGY 


into  secondary  specialized  tissues  (muscular  tissue,  nerve 
tissue,  etc.)  for  one  function  or  another  —  the  simple  life 
processes  of  Hydra  are  adequately  provided  for  by  the 
specialization  of  isolated  cells  or  small  cell  groups  within 
ectoderm  and  endoderm.  (Fig.  65.) 

The  bodies  of  all  animals  above  the  Coelenterates  are  built 
up  of  three  primary  layers,  which,  as  development  of  the 


Fia.  65.  —  Hydra.  Transverse  section,  highly  magnified.  Outer  layer  of  cells, 
ectoderm;  inner  layer,  endoderm.  Between,  mesogloea,  represented  by  a  line.  (After 
Shipley  and  McBride.) 

individual  proceeds,  give  rise  to  the  secondary  tissues  and 
thereby  form  a  relatively  complex  body.  This  third  primary 
layer,  known  as  the  mesoderm,  typically  is  developed,  as  we 
have  described  earlier,  from  the  endoderm  and  comes  to 
occupy  the  position  held  by  the  mesogloea  of  Hydra;  that  is, 
between  the  ectoderm  and  the  endoderm. 

The  development  of  the  mesoderm  is  the  key  to  the  ad- 
vance in  body  organization  of  higher  animals,  because  it 
makes  possible  a  radical  change  in  plan  that  involves  the 
establishment  of  a  body  cavity,  or  COELOM,  in  which  are  dis- 


THE   ANIMAL   BODY  121 

posed  many  of  the  chief  organs  and  organ  systems.  Accord- 
ingly the  Coelentrates,  since  they  lack  the  coelom,  are  often 
referred  to  as  ACOELOMATBS,  and  the  animals  above  the 
Coelenterates,  since  they  possess  the  coelom,  are  known  as 
the  COELOMATES.  The  difference  in  structure  can  best  be 
made  clear  by  comparing  the  body  plan  of  a  higher  Inverte- 
brate, such  as  the  common  Earthworm,  with  that  of  Hydra. 

C.   EARTHWORM 

Whereas  the  Hydra  body  is  essentially  a  single  tube  com- 
posed of  two  layers  of  cells  surrounding  the  enteric  cavity, 
the  body  of  the  Earthworm  is  built  on  th^e  plan  of  a  tube 
within  a  tube  —  the  outer  tube  forming  the  body  wall,  and 
the  inner,  the  wall  of  the  ALIMENTARY  CANAL.  The  walls  of 
these  tubes  become  continuous,  or  merge  into  each  other  at 
both  ends,  and  thus  together  they  enclose  a  space,  the  coelom. 
Or,  to  state  it  another  way:  the  outer  tube,  or  body  wall, 
surrounds  a  space,  the  coelom:  through  the  coelom  runs  a 
second  tube,  the  alimentary  canal,  which  opens  to  the  ex- 
terior at  either  end  forming  the  mouth  and  anus.  (Fig.  66.) 

The  coelom  of  the  Earthworm  is  divided  by  a  large  number 
of  transverse  partitions,  called  SEPTA,  which  extend  from  the 
inner  surface  of  the  body  wall  to  the  outer  surface  of  the 
alimentary  canal.  The  result  is  that  the  worm's  body  cavity 
is  not  a  continuous  space  running  from  one  end  of  the  ani- 
mal to  the  other,  but  consists  of  a  linear  series  of  chambers 
through  the  center  of  which  runs  the  alimentary  canal.  The 
limits  of  these  chambers  are  indicated  on  the  outside  of  the 
worm  by  a  series  of  grooves  which  encircle  the  body  wall. 
In  short,  the  body  is  made  up  of  a  series  of  essentially  similar 
units  known  as  METAMERES,  and  thus  affords  a  simple  exam- 
ple of  METAMERISM,  which  is  a  characteristic  of  all  the  higher 
animals.  (Fig.  67.) 


122 


FOUNDATIONS    OF   BIOLOGY 


Many  of  the  chief  organs  of  the  Earthworm  are  developed 
as  outgrowths  from  the  walls  enclosing  the  coelom,  so  that 
it  is  in  this  cavity  that  we  find,  for  example,  the  main  parts 
of  the  organ  systems  devoted  to  circulation,  excretion,  and 
reproduction,  as  well  as  the  nervous  system.  Moreover,  the 


71 


FIG.  66.  —  Diagrams  of  the  body  plan  of  the  Earthworm.  A  and  C,  longitudinal 
sections;  B,  transverse  section,  a,  aortic  loops  of  the  blood  vascular  system;  al,  ali- 
mentary canal;  an,  anus;  e.g.,  brain  (cerebral  ganglion) ;  coe,  coelom;  cv,  blood  vessels 
(parietal)  to  body  wall;  ds,  partitions  (septa)  between  the  segments;  d.v.,  dorsal  blood 
vessel;  ra,  mouth;  n,  nephridia;  o,  ovary;  o.d.,  oviduct;  s.i.,  ventral  blood  vessel. 
{From  Sedgwick  and  Wilson.) 

organs  are  symmetrically  disposed  with  respect  to  the  long 
axis  of  the  body  which  passes  from  mouth  to  anus.  For 
instance,  the  chief  blood  vessels  and  the  nerve  cord  lie  in  the 
long  axis  and  extend  from  end  to  end,  while  the  organs  of  the 
excretory  and  reproductive  systems  are  disposed  in  pairs  on 
either  side  of  this  axis.  Thus  there  may  be  passed  through  the 
main  axis  a  single  plane  which  divides  the  body  into  sym- 
metrical halves,  each  of  which  is  a  'mirror  picture'  of  the 


123 


124 


FOUNDATIONS    OF   BIOLOGY 


other.  The  main  axis,  therefore,  extends  from  the  mouth 
(ANTERIOR  END)  to  the  anus  (POSTERIOR  END)  ,  and  the  plane 
which  divides  the  body  into  right  and  left  sides  passes  through 
the  upper  (DORSAL)  and  lower  (VENTRAL)  side.  This  general 


dors,  v 


typh 


neph 


hep 


FIG.  68.  —  Transverse  section  through  the  middle  region  of  the  body  of  the  Earth- 
worm, circ.  mus,  circular  muscle  fibers;  coel,  coelom;  cut,  cuticle;  dors,  v,  dorsal  blood 
vessel;  epid,  epidermis;  ext.  neph,  external  opening  of  nephridium;  hep,  gland  cells; 
long,  mus,  longitudinal  muscles;  neph,  nephridium;  nephrost,  internal  opening  of  ne- 
phridium; n.  co,  nerve  cord;  set,  setae;  sub.  n.  vess,  subneural  vessel;  typh,  typhlosole; 
•cent,  v,  ventral  vessel.  (From  Parker  and  Haswell,  after  Marshall  and  Hurst.) 

disposition  of  organs  is  known  as  BILATERAL  SYMMETRY  and 
is  characteristic  of  all  higher  animals. 

The  body  of  the  Earthworm  is  radically  different  from  that 
of  Hydra,  exhibiting  as  it  does  such  essential  features  as 
coelom,  bilateral  symmetry,  and  metamerism,  which  are 
adopted  by  higher  animals  as  the  basic  plan  of  organization. 


THE   ANIMAL   BODY  125 

It  is  important  in  this  connection  to  understand  how  these 
modifications  are  related  to  the  third  primary  germ  layer,  or 
mesoderm,  which,  as  we  have  stated,  plays  a  part  in  the 
development  of  all  forms  above  Hydra.  For  the  sake  of  con- 
creteness  we  shall  describe  the  development  of  the  Earth- 
worm from  the  fertilized  egg  to  the  establishment  of  the  general 
body  plan,  though  it  must  be  borne  in  mind  that  in  no  two 
species  of  animals  is  the  process  of  development  identical. 

After  fertilization,  the  egg  of  the  worm  proceeds  to  divide 
first  into  two  cells,  then  four  cells,  eight  cells,  and  so  on,  with 
more  or  less  regularity,  until  a  condition  is  attained  in  which 
many  relatively  small  cells  are  arranged  about  a  central 
cavity.  This  stage  of  the  embryo  will  be  recognized  as  the 
blastula.  (Fig.  69.) 

The  various  cells  of  the  blastula  appear  essentially  the 
same  except  that  those  at  one  end  are  somewhat  larger  than 
at  the  other.  The  larger  cells  now  sink  into  and  nearly  ob- 
literate the  central  cavity  of  the  blastula,  thus  forming  a 
typical  gastrula  stage  composed  of  two  layers  of  cells,  ec- 
toderm on  the  outside  and  endoderm  on  the  inside.  The 
infolded  endoderm  pouch  (ENTERIC  POUCH)  enclosing  the  en- 
teric cavity  eventually  becomes  the  main  part  of  the  alimen- 
tary canal  of  the  worm,  its  present  opening  to  the  exterior 
(BLASTOPORE)  forming  the  mouth.  The  ectoderm  is  destined 
to  form  the  skin,  or  outer  layer  of  the  worm's  body. 

While  these  two  primary  germ  layers  are  being  established, 
the  developing  embryo  shows  the  rudiments  of  the  third 
primary  germ  layer  (mesoderm)  in  the  form  of  two  cells 
(POLE  CELLS)  which  leave  their  original  position  in  the  wall 
of  the  embryo  and  take  up  a  place  between  the  ectoderm 
and  endoderm ;  that  is,  in  the  remnant  of  the  cavity  of  the 
blastula  which  the  invagination  process  during  gastrulation 
has  not  completely  obliterated.  Here  the  pole  cells,  by  di- 


FIG.  69.  —  Diagrams  of  stages  in  the  development  of  the  Earthworm.  A,  blastula  (surrounded 
by  a  membrane) ;  B,  section  of  a  blastula  showing  blastocoel  and  one  of  the  primary  cells  (pole 
colls)  of  the  mesoderm;  C,  later  blastula  with  developing  mesoderm  bands;  D,  start  of  gastrula- 
tion;  E,  lateral  view  of  gastrula  showing  invagination,  which  as  it  proceeds  leaves  the  mesoderm 
bands  on  either  side  of  the  body  as  indicated  by  the  cells  represented  with  dotted  outline; 

F,  section  of  E,   along  the  line  »S-<S  to  show  pole  cells,  mesoderm  bands,  and  enteric  cavity. 

G,  later  stage  showing  cavities  in  the  mesoderm  bands,  H,  the  same  (G)  in  cross  section;   7,  dia- 
gram of  a  longitudinal  section  of  a  young  worm  after  formation  of  mouth  and  anus;    J,  the  same  in 
cross  section;   K,  later  stage  in  cross  section,    nl,  alimentary  canal;   an,  anus;  ar,  enteric  cavity; 
coe,  coelom;  ec,  ectoderm;  en,  endoderm;  m,  primary  mesoderm  cells  or  pole  cells;  m2,  mesoderm; 
mh,  mouth;  n,  nerve    cord;   s,  cavity  of  segment;   sc,  blastocoel;  sm,  somatic  layer  of  meso- 
derm which  with  the  ectoderm  forms  the  body  wall;  splm,  splanchnic  layer  of  mesoderm  which 


THE    ANIMAL    BODY  127 

vision,  form  on  either  side  of  the  enteric  pouch  a  linear 
series,  or  band,  of  mesoderm  cells.  These  MESODERM  BANDS 
gradually  increase  in  size  and  spread  out  until  finally  they 
unite  above  and  below,  that  is  encircle,  the  enteric  pouch. 
Thus  they  form  a  continuous  mesoderm  layer  between 
ectoderm  and  endoderm.  Simultaneously  with  the  growth 
of  the  mesoderm  bands  to  form  a  definite  middle  layer,  a 
linear  series  of  spaces  appears  in  each  band  which  presages 
the  future  segmentation  of  the  worm's  body.  These  cavities 
increase  in  size  and,  when  the  bands  unite  around  the  enteric 
pouch,  the  corresponding  cavities  of  each  band  also  become 
continuous  in  the  same  regions. 

In  this  way  the  mesoderm  becomes  divided  up  into  what 
are  essentially  two  cellular  layers,  an  outer,  or  SOMATIC  LAYER, 
next  to  the  ectoderm,  and  an  inner,  or  SPLANCHNIC  LAYER,  in 
contact  with  the  endoderm.  The  space  between  these  layers 
of  the  mesoderm  is  the  body  cavity,  or  coelom.  The  coelom, 
however,  is  not  a  continuous  cavity  from  one  end  of  the  em- 
bryo to  the  other,  because  the  mesodermal  cells  which  sepa- 
rated the  linear  series  of  cavities  in  the  respective  mesodermal 
bands  persist.  These  cells  form  a  regular  series  of  connecting 
sheets  of  tissue  between  the  two  mesoderm  layers  and  thus 
divide  the  body  of  the  worm  into  a  series  of  essentially  similar 
segments,  or  metameres,  the  limits  of  which  are  indicated  on 
the  outside  by  a  series  of  grooves  which  encircle  the  worm's 
body. 

While  these  processes  are  transforming  the  two-layered 
gastrula  into  an  embryo  composed  of  three  primary  layers, 
and  exhibiting  metameric  segmentation,  coelom,  etc.,  —  in 
short,  the  'tube  within  a  tube'  body-plan  characteristic  of 
higher  forms  —  the  embryo  is  gradually  increasing  in  size  and 
elongating.  The  mouth,  representing  the  blastopore,  remains 
at  one  end,  which  is  therefore  designated  as  anterior,  while 


128  FOUNDATIONS    OF    BIOLOGY 

growth  is  chiefly  in  the  opposite  direction  or 'toward  the  pos- 
terior. At  this  end  (the  blind  end  of  the  enteric  pouch  formed 
at  gastrulation)  an  opening  to  the  exterior,  the  anus,  is  formed 
so  that  the  enteric  pouch  now  communicates  with  the  ex- 
terior at  both  ends  and  becomes  the  alimentary  canal.  Thus 
antero-posterior  differentiation  is  clearly  established. 

A  cross  section  perpendicular  to  the  main  axis  of  the  devel- 
oping worm  at  this  stage  presents  the  appearance  of  a  circle 
within  a  circle.  The  smaller  circle  surrounds  the  enteric 
cavity  and  is  the  wall  of  the  alimentary  canal.  It  is  separated 
by  a  space,  the  coelom,  from  the  larger  circle,  or  body  wall. 
Moreover,  each  of  these  circles  is  composed  of  two  tissue 
layers:  the  alimentary  canal,  formed  internally  of  endoderm 
and  externally  of  mesoderm;  and  the  body  wall,  internally 
of  mesoderm  and  externally  of  ectoderm.  Thus  the  coelomic 
cavity  is  entirely  enclosed  by  mesoderm. 

It  is  from  these  four  layers  of  cells  (ectoderm,  somatic  and 
splanchnic  mesoderm,  and  endoderm)  that  all  of  the  tissues 
and  organs  of  the  adult  worm  arise  through  thickenings, 
foldings,  outgrowths,  etc.  For  example,  the  nervous  system 
is  formed  by  the  ingrowth  of  a  thickened  region  of  the  ecto- 
derm; the  blood  vascular  system  develops  by  a  specialization 
of  cells  throughout  the  mesoderm;  while  the  reproductive 
system  first  appears  as  thickenings  of  the  somatic  mesoderm 
which,  as  development  proceeds,  becomes  largely  separated 
from  it  as  independent  organs  in  the  coelom.  In  general,  itt 
may  be  said  that  in  all  the  higher  animals  the  ectoderm  forms 
the  outer  skin  and  nervous  system;  the  endoderm  supplies 
the  lining  membrane  of  the  major  part  of  the  alimentary 
tract;  while  the  mesoderm  contributes  muscles,  blood  vessels, 
reproductive  organs,  and  the  membrane  lining  the  coelom.  i 
This  similarity  in  origin  of  the  organ  systems  throughout 
the  animal  series  above  Hydra  and  its  allies  is  of  the  highest 


THE   ANIMAL   BODY  129 

significance,  because  it  indicates  a  basic  structural  identity  in 
the  body  plan  of  all  these  forms.  It  is  exhibited  in  the 
developmental  process  in  each  generation,  even  though  the 
adult  body  in  the  various  groups  differs  widely  in  form  and 
arrangement  of  organs.  Such  a  state  of  affairs  clearly  sug- 
gests a  genetic  relationship  throughout  the  whole  animal 
series  —  the  origin  of  the  diverse  forms  by  evolution. 

4      D.   CRAYFISH 

Bearing  in  mind  the  general  plan  of  organizatioi^and 
development  of  the  body  of  the  Earthworm,  we  must  next 
consider  briefly  the  main  principle  underlying  the  changes 
in  this  plan  which  give  rise  to  many  of  the  diverse  forms 
among  the  higher  Invertebrates.  This  principle  appears  to 
be  chiefly  a  specialization  of  the  individual  segments  so  that 
the  body,  instead  of  consisting  of  a  large  number  of  essen- 
tially similar  metameres,  is  formed  of  a  linear  series  of  meta- 
meres,  many  of  which  are  quite  different  from  the  rest. 
Moreover,  by  the  partial  or  complete  fusion  of  two  or  more 
metameres  and  the  suppression  of  segmentation,  definite 
regions  of  the  body  are  delineated.  This  principle  is  well 
illustrated  by  animals  of  the  group  known  as  the  ARTHRO- 
PODA,  or  'jointed-footed'  Invertebrate^  such  as  Lobsters,  In- 
sects, Millipedes,  and  Spiders.  Altogether  the  Arthropoda 
comprises  nearly  half  a  million  living  species. 

The  body  of  a  primitive  Arthropod  differs  from  that  of  the 
Earthworm  chiefly  in  the  reduction  of  |,hp  number  of  seg- 
ments  and  the  development  of  paired  jointed  appendages  as^ 
^outgrowths  from  the  body  in  each  sepnerit.  (Fig.  70.)  From 
such  a  type  all  the  multitude  of  diverse  forms  of  Arthropod 
bodies  can  be  derived.  For  instance,  in  the  CRAYFISH,  which  is 
essentially  a  fresh-water  Lobster,  the  body  consists  of  nine^ 

to  5  together  form  the 


130  FOUNDATIONS   OF   BIOLOGY 

HEAD;  segments  6  to  13.  the  THORAX:,  and  segments  14  to  19, 
the  ABDOMEN.  In  other  words,  by  the  coalescence  or  com- 
plete fusion  of  certain  segments,  the  body  has  become  divided 
jnto  more  or  less  distinct  regions.  (Fig.  71.)  Also,  the  primi- 
tive  locomotor  appendages  of  the  respective  segments  have 
become  modified  into  organs  for  the  performance  of  widely 
different  functions:  those  of  the  head,  as  sensory  organs, 
jaws,  etc.;  those  of  the  thorax,  as  organs  for  grasping,  offense 
and  defense,  and  walking;  and  those  of  the  abdomen  for 
swimming,  etc.  Thus  change  in  structure  has  gone  on  hand 


FIG.  70.  —  Diagrammatic  representation  of  the  structure  of  a  primitive  Arthropod 
in  which  very  little  specialization  of  the  segments  has  occurred.  A,  eye;  D,  digestive 
tract;  F,  antenna;  G,  jointed  appendages;  H,  dorsal  blood  vessel;  M,  mouth  append- 
ages; N,  ventral  nerve  cord  with  ganglia;  S,  mouth;  Sk,  chitinous  exoskeleton;  OS, 
cerebral  ganglion;  Us,  suboesophageal  ganglion.  (After  Schmeil.) 

in  hand  with  change  in  function,  so  that  although  there  is  no 
superficial  resemblance  between  the  jaws  of  the  Crayfish  and 
the  legs  employed  for  swimming,  nevertheless  a  study  of  their 
development  shows  beyond  doubt  that  they  owe  their  origin 
to  modifications  of  one  primary  type.  Accordingly  thp,  vari- 
ous appendages  are  said  to  be  HOMOLOGOUS,  signifying  a 
fundamental  similarity  of  structure  based  on  descent  from 
ji  common  antecedent  form.  .(Fig.  72.) 

On  the  other  hand,  organs  of  dissimilar  fundamental 
structure,  which  nevertheless  perform  the  same  function,  are 
called  ANALOGOUS.  In  the  group  of  the  Arthropods  known 
as  the  Insects,  the  series  of  head  appendages  and  the  legs  are 
homologous  with  those  of  the  primitive  Arthropod  type, 


131 


132 


FOUNDATIONS   OF   BIOLOGY 


while  the  wings  are  new,  unrelated  structures  and  not  modi- 
fications of  the  primitive  serial  appendages  of  the  ancestral 
form.  However,  as  we  shall  see  later,  the  wing  of  a  Bird  and 


5.**M..HI.  6.lfM«,lliped  7>r,MM.llhp. 


**  8.  y.d  Leg 


O.CopuUrory  Organs  10. Swimming  Poor 


FIG.  72.  —  Typical  appendages  of  a  Crayfish.  All  have  been  derived  from  a  simple 
biramous  appendage  similar  to  the  swimming  foot  (10).  Protopodite,  endopodite,  and 
exopodite  are  homologous  throughout  the  series,  en,  1-5,  parts  of  endopodite;  ep, 
epipodite;  ex,  exopodite;  fl,  parts  of  antennule;  g,  gill;  pr,  1-2,  parts  of  protopodite. 
(From  Parker  and  Haswell,  after  Huxley.) 

the  arm  of  Man  are  homologous,  while  the  wing  of  an  Insect 
and  the  wing  of  a  Bird  are  analogous  structures.  One  of  the 
chief  tasks  of  the  branch  of  biology  known  as  COMPARATIVE 
ANATOMY  is  to  discover  the  various  parts  of  plants  or  of  animals 


THE   ANIMAL   BODY  133 

which  are  homologous  and  to  study  the  modifications  which 
are  associated  with  change  of  function.     (See  p.  353.) 

We  have  considered  the  principle  of  specialization  and 
fusion  of  the  segments  of  the  higher  Arthropods  in  so  far 
as  it  affects  external  structures,  but  profound  modifications 
of  the  internal  organs  also  occur.  In  the  first  place,  the 
partitions  between  the  various  segments  which  are  present 
in  the  Earthworm  have  disappeared  in  the  Crayfish.  Again, 
the  alimentary  canal  of  the  Earthworm  is  a  nearly  straight 
tube  extending  through  the  coelom,  with  relatively  slight 
modifications  in  certain  segments  for  the  elaboration  of  the 
food  material  as  it  passes  along  from  mouth  to  anus ;  while  in 
the  Crayfish  we  see  the  accentuation  of  such  modified  regions, 
and  the  development  of  large  outpocketings  which  are  spe- 
cialized for  the  formation  of  chemical  substances  to  DIGEST 
the  food  material.  That  is,  to  change  the  food  into  a  soluble 
form  so  that  it  can  pass  through  the  cellular  membrane  which 
lines  the  digestive  tract  and  thus  actually  pass  to  the  circula- 
tory system  for  distribution  to  the  tissues  of  the  animal. 

As  a  final  illustration  we  may  take  the  nervous  system. 
In  the  Earthworm  this  consists  of  a  nerve  cord  which  runs 
along  the  body  in  the  mid-ventral  line  below  the  digestive 
tract.  At  the  anterior  end,  it  bifurcates  into  commissures 
which  encircle  the  digestive  tract  and  unite  above  in  a  rela- 
tively large  body  of  nervous  tissue  which  constitutes  the 
cerebral  ganglion,  or  BRAIN.  In  each  segment  the  nerve  cord 
also  is  somewhat  enlarged  to  form  masses  of  nerve  tissue 
(GANGLIA)  from  which  nerves  pass  to  the  organs  in  the  vicinity. 
The  nervous  system  of  the  Crayfish  exhibits  the  same  general 
plan  as  that  of  the  Earthworm,  but  certain  modifications  have 
been  brought  about  by  the  coalescence  of  segments  in  the 
region  of  the  head  and  thorax.  This  process  has  resulted  in 
the  union  of  the  segmental  ganglia  in  this  region  into  larger 


134 


FOUNDATIONS   OF   BIOLOGY 


ganglionic  masses.  The  brain  of  the  Crayfish,  for  example, 
comprises  the  primitive  ganglia  of  the  segments  which  have 
coalesced  to  form  the  head.  (Fig.  73.) 


FIG.  73. —  Diagram  of  the  general  plan  of  the  anterior  portion  of  the  central 
nervous  system  of  an  Earthworm  and  a  Crayfish,  o,  brain  (cerebral,  or  supraoesopha- 
geal,  ganglion);  b,  nerve  commissures,  encircling  the  pharynx  (shown  in  section);  c, 
suboesophageal  ganglion;  d,  ganglia  of  the  ventral  nerve  cord,  with  nerves  emerging. 

We  have  now  considered  the  fundamental  body  plan  of 
Hydra,  Earthworm,  and  Crayfish.  These  Invertebrate  types 
afford  an  excellent  background  for  a  proper  understanding 
of  the  body  structure  of  the  Vertebrate  groups.  Hydra 
exhibits  the  simple  two-layered  condition  (ectoderm  and 
endoderm)  which  is  a  transient  phase  in  the  early  develop- 


THE   ANIMAL   BODY  135 

ment  of  higher  forms.  The  Earthworm  is  of  particular 
value  since  it  illustrates  bilateral  symmetry,  an  alimentary 
canal  opening  to  the  exterior  by  an  anterior  mouth  and  a 
posterior  anus,  metameric  segmentation,  coelom,  definite 
organ  systems  for  various  functions,  and,  finally,  the  part 
played  in  development  by  the  mesoderm.  The  Crayfish 
shows,  in  simple  form,  certain  general  principles  underlying 
the  modification  of  the  Earthworm  type,  which  involve  the 
specialization  of  various  regions  in  connection  with  the  change 
of  functions  of  the  parts  to  fulfil  more  complex  life  conditions. 
The  reader,  however,  must  be  cautioned  against  supposing 
that  there  is  a  sort  of  progression  through  all  the  series  of 
lower  animals  up  to  the  Vertebrates.  We  have  selected  from 
the  groups  of  Invertebrates  certain  types  which  illustrate 
several  of  the  fundamental  structural  principles  which  are 
to  be  found  in  the  Vertebrate  body,  but  there  are  other  In- 
vertebrate groups  that  exhibit  body  plans  which  depart 
widely  from  the  types  described.  The  consideration  of  the 
morphology  of  the  groups  which  comprise  such  forms  as 
the  Tapeworms,  Rotifers,  Sea  Urchins,  Oysters,  etc.,  would 
but  tend  to  obscure  those  principles  which  are  requisite  for  a 
proper  interpretation  of  the  structure  and  functions  of  th< 
Vertebrates,  including  Man. 

E.   VERTEBRATES 

The  Vertebrates  form  one  of  the  most  clearly  defined  divi- 
sions of  the  animal  kingdom  and  include  all  the  larger  and 
more  familiar  animals  —  Fishes,  Amphibians,  Reptiles,  Birds, 
and  Mammals  —  so  that  in  the  popular  mind  the  words 
animal  and  Vertebrate  are  essentially  synonymous.  (Figs. 
82-87.) 

A  Fish,  as  every  one  knows,  is  an  aquatic  backboned  animal 
which  breathes  by  means  of  gills  and  moves  by  fins.  An 


136  FOUNDATIONS   OF   BIOLOGY 

Amphibian  may  be  thought  of  as  a  Fish  which  early  in  life 
—  at  the  end  of  the  tadpole  stage  —  discards  its  gills,  devel- 
ops lungs,  substitutes  five-toed  limbs  for  fins,  and  takes  up  a 
terrestrial  existence.  In  the  same  general  way,  a  Reptile  may 
be  pictured  as  an  Amphibian  which  has  relegated,  as  it  were, 
the  tadpole  stage  to  the  egg,  and  therefore  emerges  with  limbs 
and  lungs.  Birds  and  Mammals  may  be  regarded  as  deriva- 
tives of  the  reptilian  stock  which  have  transformed  the  scales 
of  the  reptile  into  feathers  and  hair  respectively,  and  have 
developed  a  special  care  for  their  young;  the  Birds  by  incu- 
bation of  the  eggs  and  the  Mammals  byretention  of  the  young 
essentially  as  parasites  within  the  body  of  the  female  until 
birth  occurs.  It  will  be  appreciated,  of  course,  that  other 
important  characteristics  —  some  of  which  will  be  apparent 
as  we  proceed  —  delineate  these  chief  Vertebrate  groups; 
but  there  is,  in  fact,  less  diversity  in  structure  among  the 
Vertebrates  as  a  whole  than  is  present,  for  example,  in  the 
one  subdivision  of  the  Arthropods,  the  Crustacea,  of  which 
the  Crayfish  is  a  member.  Accordingly  we  shall  confine  our 
attention  largely  to  a  description  of  the  structure  and  physi- 
ology of  an  'ideal'  Vertebrate,  and  mention  incidentally,  so 
far  as  possible,  the  chief  modifications  of  general  significance 
which  appear  in  the  different  groups. 

1.   Body  Plan 

The  ideal  Vertebrate  body  is  more  or  less  cylindrical  in 
form,  and  is  bilaterally  symmetrical  with  respect  to  a  plane 
passed  vertically  through  the  main  axis  which  extends  from 
the  anterior  to  the  posterior  end.  Three  regions  of  the  body 
may  be  distinguished,  HEAD,  TRUNK,  and  TAIL.  The  head 
forms  the  anterior  end  and  contains  the  brain,  eyes,  ears,  and 
nostrils,  as  well  as  the  mouth  and  throat.  On  either  side  of 
the  head  is  a  series  of  openings,  or  GILL  SLITS,  leading  into  the 


THE   ANIMAL   BODY 


137 


SPINAL  CORD 


NEURAL  CANAL 

NOTOCHORD 


BRAIN 


ORAL  CAVITY  / 

GILL  SLITS  H 


COELOM 


CLOACA 


SPLEEN        URINARY  BLADDER 


BILE  DUCT 
PANCREAS 


Fia.  74.  —  Diagrammatic  longitudinal  section  of  an  ideal  Vertebrate  (female). 
(From  Hegner,  after  Wiedersheim.) 


sp.c 


CTl 


FIG.  75.  —  Diagrammatic  transverse  section  through  the  trunk  of  an  ideal 
Vertebrate,  en,  centrum  of  vertebra;  coel,  coelom;  crd.  v,  cardinal  vein;  d.ao, 
dorsal  aorta;  d.f,  dorsal  fin;  d.m,  dorsal  muscles;  f.r,  fin-ray;  gon,  gonad;  int, 
intestine;  l.v,  lateral  vein;  mes,  mesentery;  ms.n.d,  mesonephric  duct;  ms.nph, 
mesonephros;  na,  neural  arch;  p.n.d,  pronephric  duct;  pr,  peritoneum,  parietal 
layer;  pr',  peritoneum,  visceral  layer;  r,  subperitoneal  rib;  r',  intermuscular  rib; 
sp.c,  spinal  cord;  t.p,  transverse  process;  v.m,  ventral  muscles.  (From  Parker 
and  Haswell.) 


138  FOUNDATIONS   OF   BIOLOGY 

throat,  which,  however,  in  air-breathing  Vertebrates  disap- 
pear before  the  adult  condition  is  attained.  The  trunk  forms 
the  body  proper  and  its  cavity,  or  coelom,  contains  the  ali- 
mentary canal,  opening  to  the  exterior  by  the  anus,  as  well 
as  the  chief  circulatory,  excretory,  and  reproductive  organs. 
The  tail  comprises  the  region  posterior  to  the  coelom  and 
anus.  (Figs.  74,  75.) 

In  aquatic  forms  thin  extensions  from  the  trunk  and  tail 
form  median  and  paired  FINS,  the  latter  comprising  the 
PECTORAL  fins,  situated  near  the  junction  of  head  and  trunk, 
and  the  PELVIC  fins,  just  lateral  to  the  anus.  The  pectoral 
and  pelvic  fins,  or  the  fore-limbs  and  hind-limbs  which  re- 
place them  in  all  forms  above  the  Fishes,  are  the  only  lateral 
appendages  found  in  Vertebrates. 

2.  Skin 

The  surface  of  the  body  which  comes  in  direct  contact 
with  the  environment  is  covered  by  an  integument,  or  SKIN, 
which,  though  primarily  protective  and  sensory  in  function, 
takes  part  to  a  greater  or  less  degree  in  respiration,  excretion, 
and  secretion.  Scales,  feathers,  claws,  horns,  hoofs,  nails, 
teeth,  etc.,  are  derivatives  of  the  skin.  The  skin,  unlike  that 
of  the  Invertebrates,  is  formed  of  two  layers;  an  outer  EPI- 
DERMIS derived  from  the  ectoderm,  and  an  inner  DERMIS  from 
the  mesoderm  of  the  embryo.  (Fig.  76.) 

3.   Muscles 

The  body  wall  proper  is  chiefly  composed  of  MUSCULAR 
TISSUE,  commonly  spoken  of  as  'flesh,'  which  varies  in  thick- 
ness in  different  regions  of  the  body.  In  the  mid-dorsal  re- 
gion it  'surrounds  the  CENTRAL  NERVOUS  SYSTEM  and  the 
axial  supporting  structure  (NOTOCHORD),  while  ventrally  it 
forms  the  wall  of  the  coelom.  In  the  lower  Vertebrates  and 


THE   ANIMAL   BODY 


139 


the  embryonic  stages  of  higher  forms  the  muscular  layer  is 
composed  of  segments  known  as  MYOTOMES.  But  in  the 
adult  stage  of  the  latter  this  evidence  of  Vertebrate  seg- 
mentation largely  disappears,  since  the  muscular  tissue  for 
the  most  part  assumes  the  form  of  highly  complex  longi- 
tudinal bands,  extensions  from  which  pass  into  the  paired 
appendages. 

A  muscle  consists  of  a  very  large  number  of  muscle  cells 
bound  together  by  connective  tissue  and  permeated  with 


FIG.  76.  —  Vertical  section  of  human  skin,  highly  magnified,  to  show  its  com- 
posite structure.  Co,  dermis;  SM,  Malpighian  layer  of  epidermis;  Se,  outer 
layer  of  epidermis;  G,  Gp,  blood  vessels;  H,  hair  with  sebaceous  glands  (D) ; 
N,  nerves;  NP,  sensory  endings  of  nerves;  SD,  sweat  glands  with  ducts  opening 
atSD1.  (From  Wiedersheim.) 

blood  vessels  and  nerves.  The  muscle  cells  themselves  have 
in  a  highly  developed  and  specialized  form  a  primary  attri- 
bute of  all  protoplasm,  contractility,  which  they  exhibit  by 
shortening  and  broadening  when  stimulated  by  impulses 
reaching  them  through  the  nervous  system.  Muscles,  such 
as  those  attached  to  the  bones,  in  which  contraction  can  be 
brought  about  at  will,  are  termed  VOLUNTARY  muscles,  while 
those  which  cause  most  of  the  movements  of  the  viscera  are 
known  as  INVOLUNTARY  muscles.  (Fig.  7,  E,  F.) 


140  FOUNDATIONS   OF   BIOLOGY 

4.    Coelom 

The  Vertebrate  coelom,  in  contrast  with  the  condition  in  the 
Earthworm,  essentially  comprises  only  two  chambers — a  large 
ABDOMINAL  cavity  which  contains  most  of  the  chief  viscera, 
and  a  small,  anterior,  PERICARDIAL  cavity  in  which  the  heart 
is  situated.  In  the  Mammals,  including  Man,  however,  the 
anterior  chamber,  known  as  the  THORAX,  contains  the  heart 
and  lungs  and  is  separated  from  the  abdominal  cavity  by 
a  muscular  partition,  or  DIAPHRAGM.  The  lining  membrane 
of  the  coelom  is  known  as  the  PERITONEUM  and  forms  the 
innermost  layer  of  the  body  wall.  (Figs.  74,  82-87.) 

5.  Skeleton 

The  form  of  the  Vertebrate  body  is  maintained  by  a  system 
of  supporting  and  protecting  structures,  termed  the  SKELE- 
TON. Although  various  outgrowths  of  the  skin,  such  as  scales, 
feathers,  and  hair,  form  a  part  of  the  skeletal  system  known  as 
the  EXOSKELETON  which  is  comparable  to  the  protective 
coverings  of  the  Invertebrates,  it  is  a  bony  ENDOSKELETON 
which  is  characteristic  of  the  higher  animals.  This  internal 
skeleton  which  is  largely  mesodermal  in  origin  exhibits 
such  great  diversity  and  complexity  that  its  study,  known  as 
OSTEOLOGY,  forms  a  most  important  subdivision  of  compara- 
tive anatomy.  In  the  lower  Fishes  the  endoskeleton  is  com- 
posed of  a  firm  elastic  tissue,  CARTILAGE,  or  gristle,  but  from 
the  'bony'  Fishes  to  Man  most  of  the  cartilage  becomes  ossi- 
fied: that  is,  impregnated  with  lime  salts  and  transformed 
into  BONE.  The  human  skeleton  is  formed  of  about  200 
separate  bones,  but  the  number  varies  at  different  periods 
of  life,  because  some  bones  which  at  first  are  distinct  later 
become  fused.  (Figs.  77,  81,  186.) 

While  it  is  true  that  the  bones  constitute  the  main  support- 


THE   ANIMAL   BODY 


141 


142 


FOUNDATIONS    OF   BIOLOGY 


ing  framework  of  the  body,  they  are  entirely  inadequate 
to  knit  together  the  organism  into  a  working  unit.  We  find 
therefore  various  kinds  of  CONNECTIVE  TISSUE  interwoven 
between  the  integral  parts  of  the  body.  These  tissues  form 
sheaths  about  most  of  the  organs  and  also  supply  the  con- 
necting links  between  muscle  and  muscle,  muscle  and  bone, 
and  bone  and  bone.  Skeletal  tissues,  of  which  bone,  cartilage, 
and  connective  tissue  form  the  chief  groups,  are  distinguished 
from  the  other  body  tissues  by  the  development  of  large 
amounts  of  non-living  material  in  or  between  the  component 
cells  themselves;  the  character  of  the  skeletal  tissue  being 

determined  chiefly  by  the 

Notochordal  sheath  r  , ,  .          ,   . 

invading  cartilage       nature  of  thls  matrlx' 

The  primitive  axis  of 

the  skeleton  consists  of 
a  cylindrical  cord  or  rod 
of  cells  (NOTOCHORD), 
which  lies  in  the  mid- 
dorsal  line  of  the  body 
wall  just  below  the  dorsal 
nerve  tube  (SPINAL  CORD) 
and  above  the  coelom.  In  most  Vertebrates,  however, 
the  notochord  in  its  original  form  is  only  a  temporary  struc- 
ture, being  partially  or  completely  replaced  during  later 
development  by  a  linear  series  of  cartilaginous  or  bony  ele- 
ments, known  as  VERTEBRAE,  which  form  the  VERTEBRAL 
COLUMN,  or  backbone.  This  is  the  most  characteristic  struc- 
ture of  Vertebrates  as  compared  with  Invertebrates,  or  back- 
boneless  animals.  (Figs  74,  78.) 

A  typical  vertebra  of  the  higher  animals  consists  of  a  basal 
portion,  known  as  the  CENTRUM,  and  a  NEURAL  ARCH  which 
it  supports.  These  form  a  protecting  ring  of  bone  about 
the  spinal  cord.  From  various  parts  of  the  vertebra  as  a 


Extent  of  one  vertebra 


FIG.  78.  —  Diagram  of  a  longitudinal  section 
through  a  developing  vertebral  column  to  show 
the  invasion  of  the  notochord  by  cartilage  to 
form  the  centra  of  the  vertebrae.  (From 
Walter.) 


THE    ANIMAL    BODY 


143 


ns 


whole  arise  PROCESSES  for  movable  articulation  with  its 
neighbors,  the  attachment  of  muscles,  etc.  Between  the 
vertebrae  of  the  Mammals  are 
cushions  of  cartilage  which  ab- 
sorb shock.  (Fig.  79.) 

In  some  forms,  RIBS  are  at- 
tached to  the  transverse  pro- 
cesses of  certain  vertebrae. 
These  extend  outward  and  down- 
ward within  the  body  wall,  and 
become  attached  in  the  mid- 
ventral  line  to  the  breast  bone 
(STERNUM).  Thus,  in  the  adult 

of    thp   hip-hpr    Vprtphrafps     thp  FlG-  79-~A  tvPical  human  ver- 

eS'     '  tebra   (tenth  thoracic)   viewed  from 

Series   Of   Centra  Of  the  Vertebrae  the  dorsal  surface.    C,  centrum;   lam, 

.                   .    .  ped,   neural  arch;     7i.s,   neural  spine; 

COme      tO      OCCUpy     the     position  prez>    anterior    articulating    process; 

formerly  held  by  the  notochord;    tr'  transverse  Process;    *•  neural 

canal  through  which  the  spinal  cord 
While    above,    the    neural     arches      passes.     (From  Walter,  after  Spalte- 

encircle  the  NEURAL  CANAL  con- 
taining the  spinal   cord;     and   below,  the  transverse  pro- 
cesses, ribs,  and  sternum  surround  the  anterior  portion  of 
the  coelom.     (Fig.  75.) 

The  Vertebrate  head,  containing  the  anterior  end  of  the 
alimentary  and  neural  canals,  the  brain,  and  the  chief  sense 
organs,  is  protected  in  the  lower  Fishes  by  a  case  of  cartilage. 
In  higher  forms  the  cartilage  is  replaced  by  a  bony  SKULL 
which  articulates  with  the  first  vertebra  of  the  backbone. 
JAWS,  or  supporting  structures  of  the  rnouth,  are  attached 
to  the  skull. 

The  skull,  vertebral  column,  ribs,  and  sternum  together 
comprise  the  AXIAL  skeleton,  from  which  is  suspended  the 
APPENDICULAR  skeleton,  or  bony  frame-work  of  the  paired 
appendages.  This  is  relatively  simple  in  the  anterior  (pec- 


144 


FOUNDATIONS   OF   BIOLOGY 


toral)  and  posterior  (pelvic)  paired  fins  of  Fishes,  which  merely 
act  as  paddles;  but  when  these  are  modified  into  paired  limbs 
for  progression  on  land,  the  mechanical  problems  involve  the 
development  of  complex  limb  skeletons  to  support  the  body, 
and  to  act  as  levers  for  the  limb  muscles  to  move  in  locomo- 


SCP 


HU 


PU 


dsb.S 
mils.  5 


JS 


FIG.  80.  —  Diagram  of  the  plan  of  the  Vertebrate  limbs.  A,  fore  limb  and  pectoral 
girdle;  B,  hind  limb  and  pelvic  girdle;  actb,  socket  for  femur;  CL,  clavicle  (collar 
bone) ;  en,  1-2,  middle  row  of  carpals  and  tarsals;  COR,  coracoid;  dst,  1-5,  distal  row  of 
carpals  and  tarsals;  FE,  femur  (thigh  bone) ;  FI,  fibula;  fi,  fibulare  (a  tarsal) ;  gl,  socket 
for  humerus;  HU,  humerus  (upper  arm  bone);  IL,  ilium;  int,  intermedium  (a  tarsal); 
IS,  ischium;  mlcp,  1-5,  metacarpals;  mtts,  1-5,  metatarsals;  p.cor,  procoracoid;  ph, 
phalanges;  PU,  pubis;  RA,  radius;  ra,  radiale  (a  carpal);  SCP,  scapula;  TI,  tibia; 
li,  tibiale  (a  tarsal);  UL,  ulna;  ul,  ulnare  (a  carpal).  (From  Parker  and  Haswell.) 


tion.  In  response  to  this  need  an  elaborate  series  of  bones  is 
developed  which,  in  all  cases,  however,  may  be  referred  to  a 
common  plan,  known  as  the  PENTADACTYL  LIMB  in  allusion 
to  the  five  digits  (FINGERS  and  TOES)  in  which  it  usually 
terminates.  The  limbs  are  attached  directly  or  indirectly  to 


THE   ANIMAL   BODY 


145 


jjll'fcr 

Tills;- 


146  FOUNDATIONS    OF   BIOLOGY 

the  axial  skeleton  by  groups  of  bones  which  form  respectively 
the  PECTORAL  and  PELVIC  GIRDLES.     (Figs.  80,  81,  185,  186.) 

F.   DIAGNOSTIC  VERTEBRATE  CHARACTERS 

As  a  summary  of  this  general  outline  of  the  structure  of  the 
Vertebrate  body,  we  may  emphasize  three  characters  which 
are  of  prime  diagnostic  importance. 

In  the  first  place,  whereas  the  skeletal  structures  of  Inver- 
tebrates typically  consist,  as  in  the  Crayfish,  of  an  exoskeleton 
of  hard  non-living  materials  deposited  on  the  surface  of  the 
body,  the  chief  function  of  which  is  protection,  the  Verte- 
brate skeleton  is  primarily  a  living  endoskeleton.  It  is  an 
organic  part  of  the  organism  which,  although  it  affords  pro- 
tection for  delicate  parts,  provides  adequately  for  support 
and  supplies  muscle  levers,  and  thus  makes  practicable  the 
relatively  large  bodies  of  the  higher  animals.  The  notochord 
is  at  once  the  foundation  and  axis  of  the  Vertebrate  internal 
skeleton  and  either  persists  throughout  life  as  such,  or  simply 
long  enough  to  function  as  a  scaffolding  about  which  the 
vertebral  column  is  built.  In  recognition  of  the  prime  im- 
portance of  the  notochord,  the  Vertebrates  and  their  nearest 
allies  (e.g.,  the  Tunicates  and  Amphioxus)  are  technically 
known  as  CHORDATES  (cf.  pp.  415,  416).. 

Glancing  back  at  the  Earthworm  and  Crayfish,  it  will  be 
recalled  that  the  central  nervous  system  consists  of  a  ventral 
nerve  cord  running  along  in  the  coelom  below  the  digestive 
tract,  except  at  the  anterior  end  where  it  encircles  the 
pharynx  to  form  the  brain  above.  The  position  of  the  Verte- 
brate brain  is  similar,  though  the  spinal  cord  is  not  a  'cord' 
but  a  nerve  tube,  which  lies  in  the  neural  canal  imbedded  in 
the  muscles  of  the  body  wall  above  the  digestive  tract  and, 
of  course,  outside  of  the  coelom.  Thus  the  spinal  cord  itself 
and  its  location  are  highly  characteristic. 


THE   ANIMAL    BODY  147 

A  third  fundamental  peculiarity  is  a  series  of  perforations 
or  slitr,  through  the  throat  and  body  wall.  In  the  lower 
forms  the  gill  slits  provide  an  exit  for  the  current  of  water 
entering  by  the  mouth  and,  being  richly  supplied  with  blood, 
afford  the  chief  means  of  respiratory  interchange  between  the 
animal  and  the  surrounding  medium.  In  the  higher  Verte- 
brates the  gill  slits  are  present  merely  during  a  transient 
phase  in  the  development  of  the  individual  since  the  function 
of  aerating  the  blood  is  taken  over  by  the  lungs.  (Figs.  74, 
75.) 


Hirst? 

$    %  .  -  *  «  M  & 


149 


150 


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SSifff  1 

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1111?; 

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3   S   «  g   Si   g1 

10  -c  b  5  s  o 

11 

§     bO 

I1! 
JH 

Q  M 

5;  i^,  glandular 
,  spleen;  22,  pei 
a;  fl9,  pulmona 
;  36,  ureter  (mei 
)be;  43,  optic  n< 
!  —  for  sperm  st 

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2.1 

55  llbl 

o3          08    ft*    LI    CD 

liitil 

151 


S^tftflSf 


S3 


152 


FIG.  87.  —  Diagrammatic  median  section  of  the  human  body.  1,  cranium;  S,  nos- 
trils (external  nares);  3,  mouth;  4,  internal  nostrils;  o,  pharynx;  6,  'Adam's  apple';  7, 
trachea  (to  lungs);  8,  oesophagus;  9,  sternum  (breast  bone)  imbedded  in  body  wall; 
10,  lungs;  11,  heart;  12,  thoracic  cavity  (part  of  coelom) ;  13,  diaphragm;  14,  ab- 
dominal cavity  (part  of  coelom) ;  15,  liver;  16,  stomach;  17,  large  intestine;  18,  kidney; 
19,  small  intestine;  20,  ureter;  21,  vermiform  appendix  of  large  intestine;  22,  urinary 
bladder;  23,  pubis  of  pelvic  girdle;  24,  urethra;  25,  anus;  26,  coccyx;  27,  posterior 
part  of  neural  canal;  28,  centrum  of  vertebra;  29,  neural  spines  of  vertebrae;  30, 
spinal  cord;  SI,  medulla;  32,  cerebellum;  S3,  cerebral  hemisphere. 


153 


CHAPTER  XI 
NUTRITION  IN  ANIMALS 

The  living  body  is  the  theatre  of  many  chemical  and  physical 
operations  in  line  with  those  of  the  inorganic  domain. 

—  Thomson. 

WE  have  now  considered  the  form  and  supporting  struc- 
tures of  the  body  wall  of  a  typical  Vertebrate;  in  other 
words,  the  outer  tube  which  surrounds  and  contains,  the 
viscera.  Through  this  outer  tube,  just  as  in  the  case  of  the 
Earthworm  and  Crayfish,  there  runs  from  mouth  to  anus  a 
second  or  inner  tube,  the  alimentary  canal,  which  has  been 
mentioned  incidentally  in  describing  the  various  regions  of 
the  Vertebrate  body. 

A.   THE  ALIMENTARY  CANAL 

The  entrance  to  the  alimentary  canal  is  the  mouth,  a  trans- 
verse ventral  aperture  near  the  anterior  end  of  the  head, 
which  leads  into  the  BUCCAL  CAVITY  supported  by  the  jaws. 
The  buccal  cavity  merges  into  the  PHARYNX,  or  throat,  which 
in  turn  leads  by  a  narrow  passage,  the  OESOPHAGUS,  to  the 
STOMACH.  (Figs.  74,  82-88.) 

In  the  aquatic  Vertebrates  the  region  of  the  alimentary 
canal  from  the  mouth  to  the  oesophagus  acts  as  a  common 
food  and  respiratory  passage.  The  food  passes  on  through  the 
oesophagus  to  the  stomach,  while  the  water  makes  its  exit  by 
a  series  of  perforations,  or  gill  slits,  through  the  pharynx  and 
body  wall  directly  to  the  exterior.  During  this  passage  the 
respiratory  interchange  of  gases  takes  place.  Among  the 

154 


NUTRITION   IN   ANIMALS 


155 


air-breathing  Vertebrates   the   gill   slits  persist  merely  as 
transient  embryonic  reminders  of  evolutionary  history,  their 


FIG.  88.  —  Diagram  of  the  alimentary  canal  and  its  derivatives  in  Man.  a, 
mouth;  b,  salivary  glands;  c,  pharynx,  showing  the  embryonic  position  of  five 
pairs  of  gill  pouches,  the  second  pair  probably  giving  rise  to  the  tonsils,  and  the 
third  and  fourth  to  the  thym us  glands;  d,  thyroid  gland ;  e,  trachea;  /,  thymus 
gland;  g,  lungs;  h,  oesophagus;  i,  diaphragm  —  the  muscular  partition  between 
the  thorax  and  abdomen;  j,  liver;  k,  gall  bladder;  I,  stomach;  TO,  pancreas; 
n,  small  intestine;  o,  large  intestine;  p,  vermiform  appendix;  q,  rectum  leading 
to  exterior  by  the  anus. 

function  being  taken  over  by  an  outpocketing  of  the  ventral 
wall  of  the  pharynx  into  the  body  cavity,  which  forms  the 
LUNGS.  Thus,  even  in  Man,  the  respiratory  membrane  which 


156  FOUNDATIONS    OF   BIOLOGY 

lines  the  lungs  is,  from  the  standpoint  of  development,  a 
specialized  part  of  the  epithelium  of  the  alimentary  canal. 
(Fig.  94.) 

The  STOMACH  is  the  first  stopping  place  of  food  which  has 
been  swallowed  and  where  the  work  of  the  digestive  tract 
(alimentary  canal)  essentially  begins  by  the  dissolving  action 
of  chemical  substances  (enzymes)  secreted  by  its  walls.  The 
stomach  leads  by  a  constriction  (PYLORIC  VALVE)  into  a  long 
and  usually  convoluted  INTESTINE.  The  anterior  portion  of 
this  is  known  as  the  SMALL  INTESTINE,  and  it  is  here  that  the 
major  part  of  digestion  is  accomplished  directly  or  indirectly 
by  means  of  chemical  secretions  supplied  by  its  walls  and  by 
the  PANCREAS  and  LIVER.  In  the  small  intestine  ABSORPTION 
also  begins;  that  is,  the  passage  of  the  soluble  food  materials 
through  the  wall  of  the  digestive  tract  into  the  body  proper. 
The  soluble  proteins  and  carbohydrates  are  taken  up 
directly  by  the  blood  vascular  system  and  conveyed  to  the 
liver,  while  the  fats  enter  the  lymph  vessels  which  later 
deliver  it  to  the  blood.  A  constriction  marks  the  origin  of 
the  LARGE  INTESTINE  which  continues  the  absorption  of 
water  and  carries  the  undigested  material,  or  FAECES,  to  the 
exterior  through  the  anus.  This  either  opens  into  a  terminal 
sac,  the  CLOACA,  in  which  also  are  situated  the  orifices  of 
the  urogenital  ducts,  or  directly  on  the  ventral  surface,  as 
in  Man.  (Fig.  93.) 

The  wall  of  the  alimentary  canal  consists  of  three  chief 
cellular  layers:  a  lining  epithelium,  a  connective  tissue  layer, 
and  a  muscular  layer.  The  epithelium,  however,  together 
with  its  derivatives  is  the  digestive  tract  proper  in  the  sense 
that  it  is  of  prime  functional  importance;  the  other  layers 
performing  accessory  functions  such  as  support,  conduction 
of  blood  vessels,  and  movements  of  the  canal.  (Figs.  20, 
103.) 


NUTRITION   IN   ANIMALS  157 

B.   DIGESTION 

Among  the  single-celled  animals  such  as  Paramecium, 
digestion  is  reduced  to  its  simplest  terms.  The  food  material 
enters  the  cell  and  is  acted  upon  directly  by  substances 
formed  by  the  protoplasm  (endoplasm)  in  its  vicinity.  In 
Hydra  a  special  layer  of  cells,  the  endoderm,  is  largely  de- 
voted to  digestion.  Although  some  of  the  endoderm  cells 
actually  engulf  small  particles  of  food  and  digest  them  within 
the  cell  (INTRACELLULAR  DIGESTION),  the  major  part  of 
digestion  is  brought  about  within  the  enteric  cavity  by  secre- 
tions from  the  endoderm  cells.  Digestion  of  the  latter  type 
(INTERCELLULAR)  is  characteristic  of  all  higher  animals  and 
reaches  its  full  development  in  the  Vertebrates. 

The  alimentary  canal  is  essentially  a  tubular  chemical 
laboratory  which  passes  the  food  on  by  its  own  muscular 
activity,  known  as  PERISTALTIC  CONTRACTIONS,  from  one  com- 
partment to  another.  Each  of  these  compartments,  in  turn, 
supplies  the  chemical  reagents  which  it  uses  for  changing 
the  food  into  a  soluble  form  so  that  it  can  pass  through  the 
walls  to  be  taken  up  by  the  circulatory  system  and  finally 
distributed  to  the  cells  of  the  organism  as  a  whole.  The 
complex  food  materials  which  enter  the  human  mouth  run 
the  gauntlet  of  a  whole  series  of  digestive  fluids.  The  sali- 
vary glands  in  the  mouth  secrete  an  enzyme  which  chemically 
modifies  the  starches;  the  gastric  glands  of  the  stomach 
supply  the  gastric  juice  containing  enzymes  which  act  on 
proteins,  and  free  hydrochloric  acid  which  renders  the 
stomach  contents  acid  in  reaction;  while  glands  in  the  intes- 
tinal walls,  and  the  pancreas  collectively  supply  other  enzymes 
which  act  on  proteins,  carbohydrates,  and  fats  in  a  medium 
made  alkaline  chiefly  by  certain  substances  from  the  liver. 

Turning  now  to  the  origin  of  the  chemicals  which  bring 


158 


FOUNDATIONS   OF   BIOLOGY 


LOCATION 

SECRETION 

ENZYMES 

SUBSTANCES 
CHANGED 

INTER- 
MEDIATE 
PRODUCTS 

PRODUCTS 
READY  FOR 
ABSORPTION 

Mouth 

Saliva 

Ptyalin 

Starch 

Maltose 

Stomach 

Gastric 
juice 

Pepsin 

Protein 

Proteoses 
and  Pep- 
tones 

Small 
intestine 

Pancreatic 
juice 

Amylopsin 

Starch 

Maltose 

Lipase 

Fats 

Fatty  acid 
and 
Glycerine 

~\*' 

Intestinal 
juiee 

Trypsin 

Maltase 
Sucrase 
Lactase 

Proteins 

Maltose      \ 
Cane  sugar  \ 
Milk  sugar  ) 

Amino 
acids 

Simple 
sugars 

Erepsin 

Proteoses 
and  Pep- 
tones 

Amino 
acids 

FIG.  89.  —  Chemical  activities  of  the  human  digestive  tract. 

about  the  solution  of  the  food  materials.  Every  cell  of  the 
body  receives  .^rom  the  circulatory  system  the  materials  nec- 
essary for  its  own  life,  but  some  cells  take  in  addition  sub- 
stances which  they  do  not  need  and,  after  transforming  them 
chemically,  contribute  the  product  as  a  SECRETION  for  the 
good  of  the  whole  organism.  Such  cells  may  act  more  or  less 
independently  as  UNICELLULAR  GLANDS,  but  generally,  for 
economy  of  space  and  adequate  blood  supply,  many  cells  are 
grouped  together  to  form  MULTICELLULAR  GLANDS.  This  is 
usually  brought  about  by  sinking  the  glandular  area  below 
the  level  of  the  membrane  to  which  it  really  belongs.  Such 
is  the  origin  of  complex  glands  as  the  liver  and  pancreas, 
which  are  outpocketings  of  the  wall  of  the  digestive  tract; 
the  sole  remaining  connection  in  each  case  being  a  narrow 
tube,  or  DUCT,  which  delivers  the  products  of  the  glands  to 
the  intestine.  (Fig.  90.) 


NUTRITION    IN   ANIMALS 


159 


Other  glandular  derivatives  of  the  digestive  tract  in  Man 
are  the  SALIVARY  glands?  of  the  mouth,  the  THYROID  and 
THYMUS  glands  near  the  anterior  end  of  the  oesophagus,  and 
the  GASTRIC  and  INTESTINAL  glands  imbedded  in  the  wall  of 
the  stomach  and  intestine  respectively.  As  a  matter  of  fact 


Duct 


Alveoli 


Secreting  Cells 


Capillary  Network 


FIG.  90.  —  Diagram  of  a  gland,  in  section,  together  with  the  surrounding  connective 
tissue  and  blood  vessels.    Highly  magnified.     (From  Hough  and  Sedgwick.) 

the  thymus  degenerates,  while  the  thyroid  loses  all  connection 
with  the  alimentary  canal  and  contributes  its  products  directly 
to  the  blood.  Accordingly  the  thyroid  as  well  as  a  number  of 
other  similar  glands,  are  known  as  DUCTLESS,  or  ENDOCRINE, 
glands,  and  their  products  as  INTERNAL  SECRETIONS.  (Fig.  88.) 
At  first  glance  the  complicated  digestive  system  of  the 
Vertebrate  may  seem  to  have  little  in  common  with  that  of ^  ^  ; 
the  Earthworm,  but  as  a  matter  of  fact  the  fundamental  plan 
is  the  same.  The  differences  which  are  present  are  the  re- 
sult of  an  increase  of  the  working  area  of  the  alimentary 


160  FOUNDATIONS    OF   BIOLOGY 

canal,  not  only  to  afford  greater  secretive  and  absorptive 
surface  and  a  larger  variety  and  amount  of  digestive  sub- 
stances, but  also  to  prolong  the  length  of  time  the  food  is 
subjected  to  treatment.  This  increase  in  area  has  been 
effected  by  folds  and  elevations  of  the  inner  surface  of  the 
tract;  by  outpushings  of  limited  areas  of  the  tube  to  form 
large  glands  which  in  most  cases  contribute  their  products  to 
their  point  of  origin  through  ducts;  and  by  increasing  the 
length  of  the  inner  tube  as  compared  with  the  outer  tube,  or 
body  wall,  which  results  in  throwing  the  intestine  into  vari- 
ous convolutions  within  the  body  cavity.  Thus  is  met  the 
increasingly  complex  nutritional  demands  of  more  highly 
organized  animals. 


CHAPTER  XII 

CIRCULATION    AND    RESPIRATION   IN   ANIMALS 

I  finally  saw  that  the  blood,  forced  by  the  action  of  the  left 
ventricle  into  the  arteries,  was  distributed  to  the  body  at  large, 
and  its  several  parts,  in  the  same  manner  as  it  is  sent  through 
the  lungs,  impelled  by  the  right  ventricle  into  the  pulmonary 
artery,  and  that  it  then  passed  through  the  veins  and  along 
the  vena  cava,  and  so  round  to  the  left  ventricle  .  .  .  which 
motion  we  may  be  allowed  to  call  circular.  —  Harvey,  1628. 

THE  crucial  points  of  contact  between  the  higher  animal 
and  its  environment,  in  so  far  as  the  intake  of  matter  and 
energy  is  concerned,  are  the  membranes  which  line  the 
digestive  tract  and  a  large  diverticulum  from  it,  the  lungs. 
Through  the  former  must  pass  all  the  materials  which  are  to 
be  assembled  as  integral  parts  of  the  organism  and  the  fuel 
which  is  to  supply  the  energy  for  the  vital  processes,  while 
through  the  latter  must  pass  the  oxygen  which  is  to  release 
this  energy.  Only  when  these  membranes  have  been  passed 
are  the  materials  really  within  the  body  and  at  its  disposal 
for  distribution  by  the  CIRCULATORY  SYSTEM  to  the  individual 
cells  of  the  various  organs  which  are  to  use  them.  In  addition 
to  carrying  the  fuel  and  the  oxygen,  the  circulatory  system 
must  remove  the  waste  products  of  metabolism  from  the  cells 
and  deliver  them  to  the  proper  excretory  organs,  such  as  the 
lungs  or  kidneys,  to  be  passed  to  the  outside  world.  The  cir- 
culatory system  is  therefore  the  essential  connecting  link  be- 
tween the  points  of  intake,  utilization,  and  outgo  of  materials 
—  a  distributing  system  which  in  cooperation  with  the  nerv- 
ous system  unifies  the  organs  into  an  organism. 

161 


162  FOUNDATIONS   OF   BIOLOGY 


A.   CIRCULATION  IN  THE  LOWER  VERTEBRATES 

In  the  higher  plants  the  movement  of  water  and  food  in 
solution  through  the  conducting  systems  is  effected  chiefly 
by  physical  forces  which  are,  to  a  certain  extent,  independent 
of,  though  directed  by,  the  activity  of  the  plant  cells.  In 
the  higher  animals,  on  the  other  hand,  circulation  is  brought 
about  by  an  active  system  which  forces  as  well  as  conducts 
throughout  the  body  what  is  to  all  intents  and  purposes  a 
fluid  tissue. 

Many  stages  in  the  evolution  of  this  elaborate  circulatory 
system  can  be  traced  from  the  lowest  coelomate  Inverte- 
brates —  in  which  it  consists  merely  of  a  single  cavity  or 
several  connected  cavities  filled  with  a  fluid  containing  vari- 
ous types  of  cells  —  through  forms  in  which  more  and  more  of 
the  spaces  are  replaced  by  definite  tubes  for  the  conduction 
of  the  fluid.  With  the  establishment  of  closed  vessels,  the 
contractions  of  various  organs  and  the  movements  of  the 
body  as  a  whole  can  no  longer  be  entirely  depended  on  for 
the  movement  of  the  fluid,  and  accordingly,  in  certain  regions, 
a  muscular  layer  is  developed  in  the  walls  of  the  tubes,  which 
by  rhythmic  pulsation  forces  the  fluid  along.  Thus,  for  exam- 
ple, in  the  Earthworm  there  is  a  fluid  (coelomic  fluid)  within 
the  body  cavity  which  is  forced  about  by  the  movements  of 
the  worm  and  bathes  most  of  the  internal  organs;  and  also 
a  system  of  vessels,  a  part  of  which  contracts  rhythmically 
and  distributes  the  blood  to  the  individual  cells.  (Figs.  66, 67.) 

In  the  higher  forms  a  closed  vascular  system  gradually  takes 
the  ascendency  and  becomes  what  one  ordinarily  has  in  mind 
when  speaking  of  'the  circulatory  system/  but  the  primitive 
type  of  open  system  still  functions  as  an  auxiliary  of  no  mean 
importance  even  in  Man.  The  highly  developed  Vertebrate 
circulatory  system,  therefore,  really  consists  of  two  parts. 


CIRCULATION   AND    RESPIRATION   IN   ANIMALS         163 

First,  a  closed  system  of  vessels  containing  BLOOD.  Blood  is 
a  lifeless  liquid  PLASMA  in  which  float  detached  cells,  the  red 
and  the  white  blood  CORPUSCLES.  Second,  a  series  of  spaces, 
channels,  and  vessels,  closely  associated  with  the  blood  vas- 
cular system,  which  is  filled  with  LYMPH.  Lymph  consists 
of  some  of  the  liquid  plasma  of  the  blood,  with  some  white 
corpuscles,  which  has  passed  through  the  thin  walls  of  the 
smallest  blood  vessels  and  bathes  the  individual  tissue  cells. 
The  lymphatic  system  really  acts  as  an  intermediary  between 
the  blood  and  the  tissues.  It  supplies  the  milieu  of  the  cells, 
and  finally  returns  the  materials  again  to  the  blood  vascular 
system. 

The  essential  elements  of  the  blood  vascular  system  are 
first,  a  muscular  organ  for  propulsion  of  the  blood,  the  HEART, 
which  lies,  as  has  been  mentioned,  near  the  mid-ventral  line 
in  the  anterior  part  of  the  coelom;  and  second,  tubes  which 
convey  the  blood  to  the  heart,  the  VEINS,  and  away  from  the 
heart,  the  ARTERIES.  The  arteries  divide  and  subdivide  to 
form  smaller  and  smaller  vessels  (ARTERIOLES)  which  finally 
merge  into  exceedingly  delicate  tubes  (CAPILLARIES)  that  per- 
meate the  tissues  of  the  body.  The  capillaries,  in  turn,  de- 
liver the  blood  to  VEINLETS  which  pass  it  on  through  larger 
and  larger  veins  to  the  heart.  Consequently  the  blood  flows 
in  a  circle  from  heart  to  heart  again,  through  a  closed  system 
of  vessels.  (Figs.  91,  92.) 

The  heart  represents  that  part  of  the  vascular  system  in 
which  the  power  of  rhythmic  contraction  has  concentrated, 
and  can  be  regarded  as  a  blood  vessel  whose  walls  have 
become  highly  modified  by  an  excessive  development  of  the 
muscular  layer.  In  the  lowest  Vertebrates  and  in  em- 
bryonic stages  of  higher  forms  the  heart  consists  typi- 
cally of  two  chief  chambers,  an  AURICLE  and  a  VENTRICLE, 
fitted  with  muscular  flaps,  or  VALVES,  which  allow  the  blood 


CIRCULATION   AND   RESPIRATION   IN   ANIMALS         165 


6  ^    >>'E 


fl    £  "*"* 
•"  £  "- 


•c  -±  S  ° 
-S  -a  &* 


-    - 


~  ~        S  ^  "3   8  'g 

"5  111! 


rat*,.,.,     c3    o3-(j    o<» 


166  FOUNDATIONS   OP   BIOLOGY 

to  flow  in  one  direction  only;  that  is,  from  auricle  to  ventricle. 
An  enlargement,  the  SINUS  VENOSUS,  connects  the  veins 
(VENOUS  SYSTEM)  with  the  auricle,  and  there  is  frequently 
another,  called  the  CONUS  ARTERIOSUS,  in  a  similar  position  at 
the  arterial  end.  The  heart  is  thus  essentially  a  linear  series 
of  chambers.  The  sinus  venosus  and  auricle  function  mainly 
as  reservoirs  to  fill  rapidly  the  especially  muscular  ventricle. 
The  latter,  acting  both  as  a  suction  and  force  pump,  passes 
the  blood  on  to  the  conus  arteriosus  and  from  there  to  the 
ARTERIAL  SYSTEM  as  a  whole.  For  our  purposes,  however,  we 
may  consider  the  heart  in  the  lowest  Vertebrates  (Fishes)  as 
composed  of  the  two  chambers,  auricle  and  ventricle.  (Fig.  91.) 

The  arterial  system  is  the  distributing  system  of  vessels 
which  carries  the  blood  to  all  regions  of  the  body.  Soon  after 
its  origin  at  the  heart  the  circuit  in  the  aquatic  forms  is  tempo- 
rarily interrupted  to  allow  the  blood  to  pass  through  the  GILLS 
and  exchange  carbon  dioxide  for  a  supply  of  oxygen.  To 
facilitate  this  gaseous  interchange,  the  arteries  (AFFERENT 
BRANCHIAL)  as  they  enter  the  gill  membrane  break  up  into 
smaller  and  smaller  vessels  which  finally  are  of  microscopic 
calibre  and  consist  of  but  a  single  layer  of  cells.  These  capil- 
laries, in  turn,  merge  into  larger  vessels  (EFFERENT  BRAN- 
CHIAL ARTERIES)  which  finally  lead  into  the  chief  artery  of 
the  body,  the  DORSAL  AORTA.  This  extends  along  the  median 
dorsal  line  of  the  body,  just  below  the  vertebral  column,  and 
sends  branches  to  the  various  organs. 

The  branches  of  the  dorsal  aorta,  on  reaching  the  location 
which  they  supply  with  arterial  blood,  break  up  into  capil- 
laries similar  to  those  in  the  gills,  and  pass  to  the  tissues 
the  blood  carrying  food  and  oxygen.  The  blood  receives  in 
return  various  waste  products  of  metabolism,  including  car- 
bon dioxide  and,  in  certain  cases,  absorbed  food  materials 
from  the  intestine,  and  special  secretions  chiefly  from  ductless 


CIRCULATION   AND    RESPIRATION    IN   ANIMALS          167 

glands.  The  fine  capillaries  lead  into  veinlets  and  these  into 
veins  of  constantly  increasing  calibre  which  sooner  or  later 
complete  the  circuit  by  returning  the  blood  to  the  heart. 

The  return  current,  however,  is  not  quite  so  simple  as 
would  appear  from  the  above  statement  because,  just  as  all 
the  outgoing  stream  is  interrupted  for  the  respiratory  inter- 
change in  the  gills,  so  a  part  of  the  return  current  is  tempora- 
rily side-tracked  through  the  liver.  The  veins  returning  blood 
from  the  digestive  organs  merge  to  form  the  PORTAL  VEIN 
which  proceeds  to  the  liver,  where  it  resolves  into  capillaries 
to  allow  that  organ  to  regulate  certain  of  the  blood  constit- 
uents —  in  particular,  to  store  up  sugar  after  a  meal  and 
later  dole  it  out  to  the  blood  as  needed.  These  capillaries 
then  pass  the  blood  into  the  HEPATIC  VEIN,  which  conveys 
it  toward  the  heart.  Thus  the  liver  receives  blood  from  two 
sources:  an  artery  providing  blood  primarily  for  the  use  of 
the  organ  itself  and  a  vein  (portal  vein)  delivering  blood  con- 
taining a  large  amount  of  food  material  solely  to  receive 
special  treatment  before  being  sent  back  to  the  heart  and  then 
all  over  the  body.  This  special  arrangement  for  a  venous 
blood  supply  to  the  liver  is  known  as  the  HEPATIC  PORTAL 
SYSTEM.  Moreover,  in  Vertebrates  lower  than  the  Birds,  the 
venous  blood  from  the  posterior  part  of  the  body  makes  a 
detour  through  the  capillaries  in  the  kidneys  on  its  way  back 
to  the  heart.  This  constitutes  what  is  termed  the  RENAL 
PORTAL  SYSTEM.  Therefore  in  these  forms  the  kidneys  as 
well  as  the  liver  receive  blood  from  two  sources,  an  artery 
and  a  vein.  It  will  be  noted  that  both  the  hepatic  portal  vein 
and  the  renal  portal  vein  arise  in  capillaries  and  terminate  in 
capillaries.  (Figs.  91-93.) 

Such  is  the  general  plan  of  the  blood  vascular  system  of  the 
lower  Vertebrates.  The  modifications  of  this  which  occur  in 
higher  forms  are  related  chiefly  to  changes  in  the  respiratory 


168 


FOUNDATIONS    OF   BIOLOGY 


mechanism  necessitated  by  abandoning  the  aquatic  for  the 
terrestrial  mode  of  life,  with  the  consequent  dependence  on 


FIG.  93.  —  Diagram  of  paths  of  absorbed  food  from  the  digestive  tract. 
Proteins  and  carbohydrates  by]  veins  (in  solid  black) ;  Fats  by  lymphatics 
(dotted).  (From  Conn  and  Budington.) 

the  free  oxygen  of  the  atmosphere  instead  of  that  dissolved 
in  the  water. 

B.  RESPIRATION 

As  we  have  seen,  the  essential  factor  of  respiration  is  an 
interchange  of  gases  between  protoplasm  and  the  environ- 


CIRCULATION   AND    RESPIRATION    IN    ANIMALS         169 

ment:  an  intake  of  free  oxygen  for  combustion,  and  an  outgo 
of  the  waste  products,  chiefly  carbon  dioxide.  In  the  unicel- 
lular organisms,  such  as  Sphaerella  and  Paramecium,  and  in 
simple  multicellular  animals  like  Hydra,  this  appears  to  be  a 
relatively  simple  process  since  an  elaborate  mechanism  is  not 
necessary  to  facilitate  the  interchange.  But  with  the  estab- 
lishment of  a  highly  differentiated  multicellular  body,  fewer 
and  fewer  cells  are  in  direct  contact  with  the  aerating  medium 
and  so  various  provisions  are  necessary  to  transfer  the  gases 
to  and  from  the  outer  world  and  the  individual  cells  them- 
selves. In  all  forms  the  skin  functions  to  some  extent;  in 
the  Earthworm,  in  fact,  it  acts  as  the  chief  respiratory 
membrane  since  a  profuse  supply  of  blood  vessels  to  the 
moist  surface  of  the  body  effects  a  sufficiently  rapid  gaseous 
interchange  for  the  relatively  inactive  life  of  the  organism. 
The  Crayfish  meets  the  problem  of  respiration  by  finger-form 
out-pocketings  of  the  body  wall,  the  gills:  a  method  of  bath- 
ing a  large  area  of  the  respiratory  membrane  in  the  respiratory 
medium,  the  surrounding  water.  The  Insects,  on  the  other 
hand,  instead  of  bringing  the  blood  to  the  surface,  develop  a 
network  of  tubes  which  ramify  throughout  the  body  and 
conduct  the  air  directly  to  the  various  tissues.  Among  the 
lower  Vertebrates,  as  has  been  indicated,  the  anterior  end  of 
the  digestive  tract  functions  as  a  common  food  and  respira- 
tory passage.  In  Fishes,  the  respiratory  water  current  which 
enters  the  mouth  makes  its  exit  by  way  of  the  gill  pouches 
and  gill  slits;  the  lining  of  the  pouches  —  outpocketings  of 
the  lining  of  the  alimentary  canal  —  functioning  as  the  res- 
piratory membrane.  (Fig.  94.) 

Among  the  air-breathing  Vertebrates  tnere  are  the  added 
problems  of  protecting  and  keeping  moist  the  greatly  in- 
creased respiratory  surface  which  their  active  metabolism 
demands.  Accordingly  the  gill  pouches  are  replaced  by  a 


170 


FOUNDATIONS    OF    BIOLOGY 


huge  outpocketing  from  the  alimentary  canal  into  the 
anterior  portion  of  the  coelom,  which  constitutes  the 
lungs.  This  entails,  in  turn,  a  complex  respiratory  mechanism 
so  that  the  air  within  the  lungs  may  be  changed  at  frequent 

intervals.  As  a  matter  of  fact 
one  ordinarily  thinks  of  the  move- 
ments involved  in  the  renewal  of 
the  air  in  lungs  as  respiration,  but 
from  what  has  been  said  it  is 
clear  that  neither  the  respiratory 
movements  involved  in  inhala- 
tion and  exhalation,  nor  the  inter- 
change of  gases  between  blood  and 
air  through  the  lung  membrane  is 
respiration  proper.  The  essential 
feature  of  respiration  takes  place 
throughout  the  body  when  the 
blood  trades  its  oxygen  supply  for 
carbon  dioxide  with  the  tissue  cells.  Thus  respiration  in 
the  final  analysis  is  the  securing  of  energy  from  food. 

C.   CIRCULATION  IN  THE  HIGHER  VERTEBRATES 

But  to  return  to  the  blood  vascular  system,  which  neces- 
sarily undergoes  far-reaching  modifications  as  a  result  of  the 
substitution  of  lungs  for  gills.  In  the  first  place  the  series  of 
paired  branchial  arteries,  which  formerly  supplied  the  gills, 
no  longer  break  up  into  capillaries,  but  instead  lead  directly 
into  the  dorsal  aorta,  and  accordingly  are  termed  AORTIC 
ARCHES.  Thus  Fishes  bequeath,  as  it  were,  to  higher  forms  a 
series  of  pairs  of  aortic  arches  which,  though  they  are  no 
longer  of  use  in  their  former  capacity,  appear  in  the  develop- 
mental stages.  Some  disappear  at  that  time  and  others  are 
modified  and  diverted  to  various  uses  in  the  adult.  (Fig.  95.) 


FIG.  94  —  Diagram  of  a  verti- 
cal section  through  the  head 
region  of  Fish  (above)  and  Reptile 
or  Bird  (below)  to  show  the  paths 
of  the  respiratory  currents  (a)  and 
food  (6).  See  Fig.  87. 


CIRCULATION   AND    RESPIRATION   IN   ANIMALS         171 

For  our  purpose  it  is  sufficient  to  emphasize  that  in  Man's 
body  one  branchial  arch  continues  to  carry  blood  directly 
from  the  heart  to  the  dorsal  aorta,  while  parts  of  another 
deliver  blood  from  the  heart  to  the  lungs  and  back  again  to 


D  E  F 

FIG.  95.  —  Diagram  to  show  the  transformation  of  the  six  pairs  of  primitive  gill  slit 
arteries  (aortic  arches)  in  the  ascending  series  of  Vertebrates.  A,  primitive  condition, 
embryonic;  B,  Fish;  C,  Amphibian  (Frog);  D,  Reptile;  E,  Bird;  F,  Mammal,  a, 
dorsal  aorta;  b,  ventral  artery  from  heart;  c,  internal  carotids;  d,  external  carotids; 
e,  e',  right  and  left  aortic  arches;  /,  pulmonary  arteries;  g,  g',  subclavian  arteries  to 
fore  limbs. 

the  heart.  Thus  there  is  established  a  second  current  of  blood 
through  the  heart,  which  necessitates  a  median  partition  in 
both  the  auricle  and  ventricle  in  order  to  keep  the  two  cur- 
rents separate.  In  this  way  a  four-chambered  heart  arises 


172  FOUNDATIONS   OF   BIOLOGY 

which  consists  of  right  and  left  auricles  and  ventricles.  The 
RIGHT  AURICLE  receives  blood  from  the  venous  system  of 
the  body  and  passes  it  through  the  TRICUSPID  VALVE  into  the 
right  ventricle  to  be  pumped  through  the  PULMONARY  ARTERY 
to  the  lungs.  After  traversing  the  capillaries  of  the  lungs  the 
blood  is  returned  by  the  PULMONARY  VEIN  to  the  LEFT  AURI- 
CLE, thence  through  the  MITRAL  VALVE  into  the  LEFT  VENTRI- 
CLE, which  forces  it  into  the  AORTA  and  so  on  its  way  about 
the  body  as  a  whole.  To  all  intents  and  purposes,  the  higher 
Vertebrates  have  two  hearts  which  act  in  unison  —  a  right,  or 
pulmonary,  heart  receiving  non-aerated  blood  from  the  entire 
body  and  pumping  it  to  the  lungs,  and  a  left,  or  systemic,  heart 
receiving  aerated  blood  from  the  lungs  and  delivering  it  to 
the  body  as  a  whole.  (Fig.  92,  C.) 

In  this  way  the  blood  vessels  of  the  primitive  aquatic  res- 
piratory apparatus  are  transformed  by  gradual  additions  and 
subtractions  into  the  pulmonary  system  of  the  higher  Verte- 
brates, including  Man  —  the  most  convincing  evidence  that 
nature,  whenever  possible,  turns  to  structures  at  hand  to 
construct  what  is  to  be,  and  thereby  weaves  in  the  woof  and 
warp  of  higher  forms  a  record  of  their  lowly  origin. 

The  blood  vascular  system  of  the  higher  Vertebrates,  in 
spite,  shall  we  say,  of  its  makeshift  origin,  is  a  highly  efficient 
apparatus.  Day  in  and  day  out  throughout  life  the  human 
heart,  beating  rhythmically  at  an  average  rate  of  70  times 
per  minute,  does  about  175,000  foot-pounds  of  work.  This 
power  is  expended  in  moving  the  weight  of  the  blood,  in 
imparting  to  it  the  velocity  of  its  motion,  and  in  raising  the 
pressure  in  the  aorta  and  pulmonary  artery. 

The  RATE  of  flow  is  greatest  when  the  blood  leaves  the  heart 
and  gradually  diminishes  until,  in  the  capillaries  of  both  the 
pulmonary  and  systemic  systems,  it  is  reduced  to  a  minimum. 
On  the  return  trip  from  the  capillaries  through  the  veins  the 


CIRCULATION   AND    RESPIRATION    IN    ANIMALS       173 

rate  of  flow  gradually  increases  though  it  reenters  the  heart 
at  a  slower  rate  than  it  departed.  Thus  of  the  23  seconds 
which  it  takes  a  unit  of  blood  to  make  the  complete  circuit 
in  Man,  about  two  seconds  are  spent  in  the  capillaries  —  a 
relatively  long  time  when  it  is  realized  that  the  average  length 
of  the  capillary  path  is  about  one  fiftieth  of  an  inch.  The 
principle  underlying  the  change  in  rate  is  simple.  The  blood, 
driven  throughout  its  course  by  the  same  force  —  the  heart 
beat  —  varies  in  rate  with  the  width  of  the  bed  through 
which  it  is  flowing.  Although  the  area  afforded  individually 
by  the  arteries  and  veins  is  greater  than  that  by  the  single 
capillaries,  nevertheless  the  total  area  afforded  by  the  capillary 
system  is  enormously  greater  than  that  by  either  the  arterial 
or  venous  system. 

Moreover,  since  a  liquid  in  a  closed  system  of  tubes  must 
flow  from  a  region  of  high  to  one  of  low  pressure,  the  blood 
PRESSURE  continuously  diminishes  from  heart  back  to  heart 
again.  But,  it  should  be  noted,  that  although  the  pressure  in 
the  capillaries  of  any  region  as  a  whole  is  greater  than  in  the 
veins  which  they  supply,  nevertheless  the  pressure  in  a  single 
capillary  is  very  low,  as  is  demanded  by  its  delicate  wall. 

Thus  it  is  apparent  that  in  the  capillaries  the  blood  moves 
very  slowly  under  low  pressure  — •  for  it  is  here  that  the  blood 
does  its  work.  All  the  rest  of  the  vascular  system  —  heart, 
arteries,  and  veins  —  is  arranged  to  give  the  blood  just  this 
opportunity  in  the  capillaries. 

It  is  in  the  capillaries  that  the  blood  vascular  system  turns 
over  the  work  of  distribution  to  the  lymphatic  system.  As 
has  been  said,  lymph  to.  all  intents  and  purposes  consists  of 
plasma  and  white  corpuscles  from  the  blood  which  have 
passed  through  the  thin  capillary  walls,  carrying  along  food 
materials,  oxygen,  etc.,  to  exchange  for  the  various  waste 
products  of  metabolism  of  the  cells  which  it  bathes.  Thus 


174  FOUNDATIONS    OF   BIOLOGY 

there  is  a  continuous  drainage  of  lymph  from  the  capillaries 
into  intercellular  lymph  spaces.  Some  of  the  fluid,  with  waste 
products,  etc.,  passes  immediately  into  the  capillaries  again, 
but  the  excess  passes  from  lymph  spaces  into  small  lymph 
vessels,  and  from  these  into  large  lymph  vessels.  The  latter, 
in  turn,  empty  into  the  venous  system  and  so  restore  the 
materials  to  the  blood.  (Fig.  93.) 

So  much  for  the  path  and  the  duties  of  the  liquid  tissue 
which  circulates  through  the  body.  But  clearly  some  provi- 
sion must  exist  for  regulating  the  blood  flow  in  order  to  meet 
the  varying  local  demands  of  the*~oTgsrHs  of  the  body  under 
different  physiological  conditions.  This  is  attended  to  chiefly 
by  nerve  impulses  which  are  conducted  by  a  system  of  VASO- 
MOTOR  nerves  and  bring  about  the  dilation  or  contraction  of 
the  smaller  blood  vessels  (arterioles)  leading  to  an  organ,  and 
thus  increase  or  decrease  the  volume  of  the  blood  which  it 
receives.  The  elaborate  mechanism  in  homothermal  animals, 
which  maintains  a  practically  constant  body  temperature, 
is  largely  dependent  upon  heat  distribution,  loss,  and  con- 
servation by  the  blood  vascular  system.  Since  the  total 
volume  of  blood  in  the  body  is  practically  constant,  an  extra 
supply  to  one  part  obviously  necessitates  a  reduced  supply 
to  another.  So  it  happens,  for  instance,  that  after  a  hearty 
meal  more  blood  is  concentrated  where  digestion  is  actively 
going  on,  leaving  less  for  the  other  organs  —  the  reduced 
supply  to  the  brain  resulting  in  the  proverbial  drowsiness 
at  such  times. 


CHAPTER  XIII 
EXCRETION  IN  ANIMALS 

The  ihathematically  accurate  end-reaction  of  a  chain  of 
known  and  unknown  causes  and  effects.  —  Noyes. 

PROVISIONS  for  eliminating  from  the  organism  the  waste 
products  of  metabolism  are  only  second  in  importance  to 
those  for  supplying  the  matter  and  energy  by  which  the  vital 
processes  are  carried  on.  Accordingly  we  find  the  kidneys 
devoted  solely  to  excretion;  the  gills  or  the  lungs,  largely  to 
excretion ;  and  the  skin  and  liver  acting  in  subsidiary  capaci- 
ties. In  nearly  every  case  the  essential  parts  of  the  excretory 
organ  are  gland  cells  which  select  from  the  blood  supply  at 
their  disposal  one  or  another  waste  product.  This  material 
they  secrete  in  more  or  less  changed  form  so  that  it  eventually 
leaves  the  body  as  an  excretion.  There  is  therefore  an  essen- 
tial distinction  between  an  EXCRETION,  which  represents 
chemical  waste  from  the  vital  processes,  and  the  major  part 
of  the  material  which  is  eliminated  from  the  digestive  tract 
as  FAECES.  The  latter  is  almost  entirely  indigestible  material 
taken  in  with  the  food  which  has  not  directly  contributed  to 
the  metabolic  processes  of  the  organism.  Accordingly  the 
digestive  tract  is  not  included  in  the  list  of  excretory  organs, 
though  as  a  matter  of  fact  certain  waste  products  excreted  by 
the  liver  reach  the  outside  world  with  the  faeces. 

We  have  already  emphasized  the  elimination  of  carbon 
dioxide  by  the  GILLS  or  the  LUNGS.  Here  the  cells  of  the  RES- 
PIRATORY MEMBRANES  play  essentially  a  passive  role  in  excre- 
tion, since  the  carbon  dioxide,  which  is  under  higher  tension 

175 


176  FOUNDATIONS    OF   BIOLOGY 

in  the  blood  than  in  the  water  or  air,  follows  the  physical  laws 
of  diffusion  of  gases  and  passes  from  the  blood.  In  addition  to 
carbon  dioxide,  the  blood  of  warm-blooded  animals  (Birds 
and  Mammals)  loses  a  large  amount  of  water  and  heat;  the 
amount  depending  on  the  temperature  and  moisture  of  the  air 
which  enters  the  lungs.  When  the  air  is  exhaled  its  tempera- 
ture is  essentially  that  of  the  body  and  it  is  saturated  with 
water  vapor. 

The  SKIN  in  some  of  the  lower  Vertebrates,  for  instance  the 
Frog,  is  an  exceedingly  important  excretory  organ,  because 
more  carbon  dioxide  is  eliminated  through  the  skin  than 
through  the  lungs ;  but  in  higher  forms,  including  Man,  excre- 
tion by  the  skin  is  confined  to  the  SWEAT  GLANDS.  These  take 
from  the  blood,  in  addition  to  large  quantities  of  water,  traces 
of  nitrogenous  waste  or  urea,  fatty  acids,  and  salts, which  form 
a  residue  on  the  surface  of  the  skin  when  the  PERSPIRATION 
evaporates.  (Fig.  76.) 

The  LIVER,  in  addition  to  its  various  other  functions,  aids 
in  no  small  way  in  excretion.  On  the  one  hand,  the  liver 
removes  deleterious  compounds  of  ammonia  from  the  blood 
and  transforms  them  into  urea.  Then  it  secretes  the  urea 
into  the  blood  from  which  it  is  later  removed  by  the  kidneys. 
On  the  other  hand,  the  liver  collects  other  waste  products  etc., 
from  the  blood,  which  form  the  bile.  This  passes  to  the  GALL 
BLADDER  for  temporary  storage  or  directly  to  the  intestine. 

The  KIDNEYS  are,  in  a  way,  the  chief  excretory  organs  o£ 
Vertebrates,  and  any  serious  interference  with  their  activity 
rapidly  leads  to  a  poisoning  of  the  body  with  its  own  waste 
products.  Certain  cells  of  the  kidneys  remove  the  urea  from 
the  blood  stream  which  reaches  them,  while  water  and  various 
solutes  are  drained  from  the  blood.  Aside  from  their  func- 
tional importance,  the  kidneys  are  of  considerable  interest 
to  the  comparative  anatomist  because  of  their  complicated 


EXCRETION   IN   ANIMALS 


177 


evolutionary  history  —  indeed  the  structure  of  the  human 
kidneys  is  intelligible  only  in  the  light  of  the  relatively  simple 
excretory  organs  of  Invertebrates,  such  as  the  Earthworm, 
and  of  lower  Vertebrates.  (Figs.  66,  67,  96.) 

The  chief  excretory  organs  of  the  Earthworm  consist  of 
pairs  of  coiled  tubes,  or  NEPHRIDIA,  segmentally  arranged  in 
the  coelom  on  either  side  of  the  alimentary  canal.  Each 
nephridium  begins  as  an  open  funnel  in  the  coelom  of  one  seg- 
ment, passes  through  the 
partition  to  the  next  posterior 
segment  and  there,  after  coil- 
ing, passes  to  the  ventral  sur- 
face and  opens  to  the  exterior 
by  a  pore.  Thus,  reduced  to 
its  simplest  terms,  a  nephri- 
dium is  a  tube  communicat- 
ing between  the  coelom  and 
the  outer  world,  and  afford- 
ing a  path  of  egress  for  the 
waste  matter  in  the  coelomic 
fluid.  But  the  closed  blood 
vascular  system  of  the  worm 
collects  various  waste  products  in  addition  to  the  carbon 
dioxide  which  it  delivers  to  the  skin.  Nitrogenous  waste, 
inorganic  salts,  etc.,  are  carried  to  the  coiled  part  of  the 
nephridial  tube  where  gland  cells  take  them  from  the  blood 
and  deliver  them  to  the  interior  of  the  tube  to  be  passed  out 
of  the  body.  Now  strange  as  it  may  seem,  although  the 
primitive  segmentation  of  the  coelom  has  disappeared  in  the 
Vertebrates,  nevertheless  there  are  good  grounds  for  believ- 
ing that  the  archaic,  segmentally  arranged  nephridia  have 
been  taken  over,  as  it  were,  and  made  the  basis  of  the  essen- 
tial excretory  elements  of  the  kidneys. 


FIG.  96.  —  Diagram  to  show  the  gen- 
eral structural  plan  of  a  nephridium  of 
an  Earthworm,  anterior  end  toward  the 
right,  a,  internal  opening  of  nephridium; 
b,  external  opening;  c,  capillary  network 
about  the  coiled,  glandular  portion. 


178  FOUNDATIONS   OF   BIOLOGY 

In  the  lowest  Vertebrates  the  primitive  type  of  kidney,  or 
PRONEPHROS  as  it  is  called,  consists  of  a  series  of  segmentally 
arranged  nephridia  in  the  dorsal  part  of  the  anterior  end  of 
the  coelom.  These,  however,  instead  of  opening  independ- 
ently to  the  exterior,  discharge  their  products  into  a  common 
tube  (PRONEPHRIC  DUCT)  which  passes  them  to  the  outside. 
In  higher  forms  the  pronephros  disappears,  and  its  function 
is  taken  over  by  another  series  of  nephridia  which  appear  in 
the  coelom  posterior  to  the  pronephros.  This  series  consti- 
tutes the  MESONEPHROS,  and  opens  into  the  pronephric  duct 
which  accordingly  now  is  called  the  MESONEPHRIC  DUCT.  Fi- 
nally, in  still  higher  Vertebrates  this  second  urinary  organ  is 
replaced  by  a  third,  the  kidney  proper  (METANEPHROS)  and 
its  special  duct,  the  URETER.  Thus  as  we  ascend  the  Verte- 
brate series  three  distinct  kidney  systems  appear,  in  each 
case  by  the  development  and  grouping  of  a  number  of  ne- 
phridia into  a  definitive  organ.  In  this  process  the  primitive 
communication  of  the  individual  nephridia  with  the  body 
cavity  is  lost  and  the  activity  of  the  glandular  portion  in- 
creased, until,  in  the  higher  forms,  all  the  waste  products  are 
taken  solely  and  directly  from  the  blood.  (Fig.  97.) 

It  is  therefore  apparent  that  each  of  the  relatively  large, 
compact  kidneys  of  the  higher  Vertebrates,  including  Man, 
is  to  all  intents  and  purposes  a  large  group  of  nephridia-like 
elements,  the  tubules,  bound  together  by  connective  tissue 
and  covered  with  a  protective  coat.  The  tubules  within  the 
kidney  deliver  the  materials  taken  from  the  blood  to  the 
pelvis  of  the  kidney,  from  which  it  passes  down  the  ureter 
and  on  to  the  URINARY  BLADDER  and  finally  to  the  exterior. 
(Fig.  98.) 

Such,  in  broad  outline,  is  the  historical  viewpoint  from 
which  the  kidneys  of  Man  must  be  interpreted.  As  a  matter 
of  fact,  however,  the  evolutionary  transformation  is  still 


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179 


180 


FOUNDATIONS    OF   BIOLOGY 


further  complicated  by  anatomical,  though  not  physiological, 
relations  with  the  reproductive  system.  As  will  be  pointed 
out  later,  this  neighboring  system  now  and  again  foists,  as  it 
were,  some  of  its  accessory  responsibilities  upon  parts  of  the 


FIG.  98.  —  Longitudinal  section  of  the  human  kidney.  Ct,  cortex,  or  region  in 
which  the  essential  nephridial  elements,  the  tubules,  come  into  functional  asso- 
ciation with  the  capillaries;  M,  medullary  portion,  through  which  the  tubules 
extend  to  open  on  the  summits  of  the  pyramids  (Py);  P,  pelvis  of  kidney;  Ra, 
renal  artery;  U,  ureter.  (After  Huxley.) 

excretory  (urinary)  system,  and  even  takes  over  portions  and 
makes  them  integral  parts  of  its  own  when  they  have  been 
permanently  abandoned  by  the  urinary  system  in  its  evolu- 
tionary development. 


CHAPTER  XIV 

COORDINATION  IN  ANIMALS 

It  seems  that  Nature,  after  elaborating  mechanisms  to  meet 
particular  vicissitudes,  has  lumped  all  other  vicissitudes  into 
one  and  made  a  means  of  meeting  them  all.  —  Mathews. 

SINCE  a  primary  attribute  of  protoplasm  is  irritability  — • 
the  power  of  responding  to  environmental  changes  by  changes 
in  the  equilibrium  of  its  own  matter  and  energy  —  it  is  not 
strange  that  the  cells  of  an  organism  mutually  modify  each 
other's  activities  and  reciprocal  interrelationships  have  been 
established  during  their  long  evolutionary  history.  The 
various  cells,  tissues,  organs,  and  organ  systems  are  unified 
into  an  organism  by  what  may  be  called  the  chemical  inter- 
play between  its  various  parts,  which  is  made  possible  by  the 
facilities  for  distribution  afforded  by  the  circulatory  system; 
and  also  by  the  directing  influence  of  the  nervous  system 
which  supplies  a  central  station  with  lines  for  instantaneous 
intercommunication  with  every  part  of  the  body. 

A.   CHEMICAL  COORDINATION 

It  is  only  with  the  recent  increase  in  knowledge  of  the 
general  problem  of  metabolism  that  the  far-reaching  impor- 
tance of  the  chemical  control  of  bodily  processes  has  grad- 
ually been  brought  to  the  fore.  Although  we  may  properly 
think  of  the  various  chemical  regulators,  or  HORMONES,  as 
forming  a  coordinating  system  in  so  far  as  their  collective 
action  has  such  a  result,  in  the  present  stage  of  our  knowledge 
it  is  possible  to  cite  only  the  specific  action  of  individual  hor- 

181 


182  FOUNDATIONS   OF   BIOLOGY 

mones  as  examples  of  the  general  method  of  chemical  regu- 
lation which  their  study,  ENDOCRINOLOGY,  is  revealing. 

Certain  hormones  are  secreted  by  organs  whose  sole  func- 
tion is  their  production,  such  as  the  various  endocrine  glands 
which  pour  their  secretion  directly  into  the  blood  stream. 
Others  are  elaborated  by  special  cells  imbedded  in  organs, 
such  as  the  pancreas  and  reproductive  organs,  of  which  they 
physiologically  form  no  part.  As  a  concrete  example  of  an 
endocrine  gland  we  may  select  the  THYROID  which,  as  has 
been  seen,  arises  as  an  outpocketing  of  the  digestive  tract  in 
the  neck  region  and  finally  loses  all  connection  with  its  point 
of  origin  and  becomes  a  ductless  gland.  (Fig.  88.) 

The  general  effect  of  the  thyroid  hormone  on  metabolism 
is  a  regulation  of  the  rate  of  oxidation  in  the  body.  An  excess 
of  the  substance  results  in  such  vigorous  fuel  consumption 
that  no  surplus  remains  in  the  body  to  be  stored  as  fat ;  while 
a  deficiency  in  the  glandular  secretion  results  in  a  tendency 
toward  fat  formation.  Accordingly  the  administration  of 
thyroid  extract  is  often  an  efficient  means  of  reducing  flesh 
by  increasing  the  oxidative  processes  of  the  body.  A  defi- 
ciency of  the  hormone  during  adult  life  frequently  results  in 
a  type  of  mental  deterioration  called  MYXEDEMA.  Children 
in  whom  the  development  of  the  thyroid  is  suppressed  be- 
come dwarfish  idiots  known  as  CRETINS,  while  overdevelop- 
ment of  the  gland  induces  increased  nervous  activity  and 
mental  disorders.  Feeding  with  thyroid  material  sometimes 
prevents  the  development  of  cretinism  and  cures  myxedema, 
while  a  surgical  removal  of  part  of  the  gland  may  cure  the 
nervous  instability  and  other  symptoms  due  to  an  excessive 
amount  of  the  hormone.  Goitre  is  a  pathological  enlarge- 
ment of  the  thyroid. 

Finally,  as  a  further  indication  of  the  nicety  of  the  recip- 
rocal adjustments  within  the  organism,  it  may  be  mentioned 


COORDINATION   IN  ANIMALS  183 

that  the  thyroid  gland  itself  is  subject  to  regulating  stimuli 
reaching  it  through  the  nervous  system,  as  well  as  by  a  hor- 
mone derived  from  the  PITUITARY  BODY  which  is  another 
endocrine  gland  situated  in  conjunction  with  the  lower  part 
of  the  brain.  Glimpses  of  such  interrelationships  are  being 
gradually  afforded  as  one  hormone  after  another  is  discovered. 
But  chemical  coordination,  indispensable  as  it  is  as  a  means 
of  regulating  many  of  the  processes  of  the  organism,  espe- 
cially the  slower  ones  such  as  growth,  is  entirely  inadequate 
for  the  instantaneous  correlation  of  diverse  parts  of  an  animal 
and  also  for  the  adjustment  of  the  whole  animal  to  its  sur- 
roundings. The  nervous  system  supplies  this  need  by  a  com- 
plicated arrangement  of  cellular  elements  in  which  irritability 
and  conduction  are  highly  developed.  (See  p.  206.) 

B.   COORDINATION  BY  THE  NERVOUS  SYSTEM 

In  some  unicellular  organisms  certain  portions  of  the 
protoplasm  are  especially  differentiated  for  receiving  and 
conducting  stimuli,  and  others  for  making  effective  such 
stimuli  by  contractions  of  the  whole  or  parts  of  the  cell. 
It  is  in  the  lower  Metazoa,  such  as  Hydra  and  its  allies, 
however,  that  we  find  the  establishment  of  definite  NERVE 
CELLS  some  of  which  are  specialized  for  receiving  stimuli  and 
others  for  conducting  the  excitation  to  cells  specialized 
for  contracting  (muscle  cells),  etc.  Thus  a  simple  RECEPTOR- 
EFFECTOR  system  arises  which  may  be  regarded  as  the  basis 
for  the  development  of  the  elaborate  NEURO-MUSCULAR 
MECHANISM  of  higher  forms.  Although  from  the  functional 
point  of  view  it  is  impossible  to  differentiate  between  the  re- 
ceiving and  conducting  elements  and  those  which  make  them 
effective  (muscular  system)  in  the  economy  of  the  organism, 
from  the  standpoint  of  anatomy  the  former  constitutes  a 
definite  entity,  the  NERVOUS  SYSTEM  proper.  (Figs.  99,  100.) 


184 


FOUNDATIONS    OF   BIOLOGY 


The   structural   elements   of   the  nervous  system  of  all 
animals  consist  of  cells    known  as  nerve  cells,  or  NEURONS. 


FIG.  99.  —  Diagram  of  a 
simple  type  of  receptor-ef- 
fector system,  found  in  some 
Hydra-like  animals.  It  com- 
prises receptors  (6),  or  sense 
cells,  reaching  to  the  body 
surface  (a),  with  basal  nerve 
net  (c)  connecting  with  mus- 
cle cells  (d).  (Slightly  modi- 
fied, after  G.  H.  Parker.) 


FIG.  100.  —  Diagram  of  a 
more  complex  type  of  recep- 
tor-effector system,  found  in 
some  Hydra-like  animals.  It 
comprises,  in  addition  to  the 
receptor  (6)  with  nerve  net 

(c)  and  the  muscle  cells  (e), 
another  nerve  (ganglion)  cell 

(d)  interpolated  in  the  nerve 
net.    a,  body  surface.     (After 
G.  H.  Parker.) 


In  the  lower  forms  these  cells  are  permanently  united  so  that 
they  form  NERVE  NETS  which  surround  and  permeate  the 
tissues  which  they  stimulate  to  action.  In  more  highly 


FIG.  101.  —  Diagram  of  a  primary  sensory  (s/)  and  motor  (mf)  neuron  of  the  ventral 
nerve  cord  of  an  Earthworm,  showing  their  connections  with  the  skin  (ep)  and  the 
muscles  (Im)  to  form  a  simple  reflex  arc.  cm,  circular  muscles;  ep,  epidermis;  Im, 
longitudinal  muscles;  me,  motor  neuron  cell  body  (in  a  ventral  ganglion),  with  fiber 
(mf);  se,  sensory  neuron  cell  body  with  fiber  (sf)  entering  ganglion  to  form  synapses 
with  processes  of  motor  neuron.  See  Fig.  68.  (After  G.  H.  Parker,  and  Rrtzius.) 


COORDINATION   IN  ANIMALS 


185 


\ 


developed  animals  the  net  arrangement  is  relegated  to  the 
control  of  relatively  subsidiary  functions  (Fig.  103),  while 
the  main  nervous  system  consists  of  neurons  arranged 
in  groups,  or  GANGLIA,  and  prolongations  of  the  neurons, 
or  nerve  FIBERS,  bound  together 
into  cables,  or  NERVES.  The 
neurons,  which  are  imbedded  in 
protective  sheaths  of  connective 
tissue  in  the  ganglia,  are  in 
physiological  continuity  one  with 
another  by  'transmitting  con- 
tacts/ or  SYNAPSES,  but  each 
neuron,  it  is  believed,  preserves 
its  structural  integrity.  (Figs. 
101,  102.) 

It  will  be  recalled  that  the 
first  great  structural  differentia- 
tion during  the  development  of 
a  multicellular  animal  establishes 

FIG.  102.  —  Diagram  of  stages  in 
an      OUter      ectoderm      and      inner     the    differentiation     of    nerve    cells 

endoderm,   and   thus    segregates   <">'  Jj;  £tXSE 

the    functions    Of    protection     and     animals;    B,    motor    neuron    of  the 
i  . .  .,  .  Earthworm;    C,    a    primary    motor 

general  reactions  to  the  environ-   neuron  of  a  vertebrate,    in  B  and 
ment  from  that  of  nutrition.     It  c  thenerve  imPulse  passes  from  be- 

low  upward.     (After  G.  H.  Parker.) 

is    natural    therefore    that    the 

ectoderm  should  become  the  seat  of  those  specializations 
which  have  evolved  into  the  nervous  system  and  sense 
organs.  Such  is  the  case  in  all  forms  from  the  lowest  to 
the  highest  and  thus  the  development  and  comparative  an- 
atomy of  the  nervous  system  of  Vertebrates,  in  particular, 
affords  the  most  cogent  evidence  of  the  genetic  continuity 
of  the  whole  series,  including  Man. 

In  the  development  of  a  Vertebrate  the  first  evidence  of 


A 


II 


186 


FOUNDATIONS    OF   BIOLOGY 


the  nervous  system  is  a  longitudinal  groove  in  the  ectoderm 
along  the  dorsal  surface,  which  soon  becomes  converted  into 

a  tube  by  the  apposition  and, 
finally,  the  fusion  of  its  edges. 
This  NEURAL  TUBE  then  becomes 
separated  from  and  sinks  below 
the  surface  ectoderm,  and  in  time 
forms  the  CENTRAL  nervous  sys- 
tem consisting  of  the  brain  and 
spinal  cord.  As  development  pro- 
ceeds, outgrowths  from  the  central 
nervous  system  establish  the 

PERIPHERAL    aild     the    AUTONOMIC 

(SYMPATHETIC)  nervous  systems, 
so  that  structurally  as  well  as 
physiologically  the  whole  nervous 
system  represents  a  unit;  a  single 
organ,  as  it  were,  which  seconda- 
rily becomes  closely  identified 
here  and  there  with  sense  organ, 
muscle,  or  gland,  as  the  case 
may  be. 

The    first    marked     structural 


9- 

FIG.  103.  —  Diagram  of  a  section 
(highly  magnified)  of  the  wall  of 
the  intestine  of  a  Vertebrate  to 
show  its  intrinsic  nervous  organiza- 
tion which  brings  about  the  move- 
ments of  the  tube.  The  two  plexuses 
consist  essentially  of  simple  neurons  modifications  in  the  developing 

central  nervous  system  of  Verte- 
brates are  two  constrictions  of  the 
enlarged  anterior  end  of  the  neural 
tube,  which  delineate  the  three 
primary  brain  vesicles:  FORE- 
BRAIN,  MID-BRAIN,  and  HIND-BRAIN.  Thus  very  early  in 
embryonic  development,  one  end  of  the  neural  tube  is 
molded  into  the  brain,  leaving  the  rest  to  form  the  spinal 
cord.  (Fig.  104.) 


arranged  as  nerve  nets.  a,  food 
absorbing  surface  of  the  intestine; 
b,  mucous  layer;  c,  plexus  of  neu- 
rons (submucouS) ;  d,  circular  mus- 
cle; e,  plexus  of  neurons  (my en- 
teric); /,  longitudinal  muscle;  g, 
serous  layer.  (From  Parker,  after 
Lewis.) 


COORDINATION   IN  ANIMALS 


187 


The  three- vesicle  brain  now  becomes  transformed  into  one 
of  five  vesicles  by  a  hollow  outpocketing  from  the  anterior  end 


FIG.  104.  —  Diagrams  to  illustrate  the  general  method  of  transformation  of  the 
anterior  end  of  the  neural  tube  into  the  brain.  A,  B,  C,  median  vertical  sec- 
tions; D,  dorsal  view  of  C.  a,  fore-brain;  b,  mid-brain;  c,  hind-brain;  d,  prosen- 
cephalon;  e,  diencephalon;  /,  cerebellum;  g,  medulla  oblongata;  h,  cerebral  hemi- 
spheres; i,  olfactory  lobes;  j,  pineal  body;  k,  inf undibulum. 

of  the  fore-brain  and  a  dorsal  outpocketing  of  the  hind-brain. 
In  some  of  the  lower  Vertebrates  the  brain  throughout  life 


188  FOUNDATIONS    OF   BIOLOGY 

consists  of  these  divisions,  known  as  PROSENCEPHALON, 
DIENCEPHALON,  mid-brain  or  MESENCEPHALON,  EPENCEPHA- 

LON   Or  CEREBELLUM,  and  METENCEPHALON   Or  MEDULLA 

OBLONGATA,  the  latter  merging  into  the  spinal  cord.  Usually, 
however,  the  prosencephalon  gives  rise  to  a  pair  of 

PARENCEPHALA,    Or    CEREBRAL   HEMISPHERES,    which    are    deS- 

tined  gradually  to  overshadow  in  development  all  the  other 
parts  of  the  brain  and  to  become  the  seat  of  consciousness 
as  well  as  of  the  higher  mental  life  in  general. 

Finally,  the  development  from  the  prosencephalon,  or 
from  the  cerebral  hemispheres  when  present,  of  a  pair  of 
RHiNENCEPHALA,or  OLFACTORY  LOBES, completes  the  establish- 
ment of  the  chief  brain  chambers.  The  further  changes 
which  transform  the  more  or  less  linear  series  of  vesicles 
into  the  increasingly  complex  and  compact  brain  of  higher 
forms  are  due  to  bendings,  or  FLEXURES,  and  to  unequal 
thickenings  and  outgrowths  of  the  chamber  walls.  For  in- 
stance, the  upper  and  lower  surfaces  of  the  diencephalon  give 
rise  to  the  PINEAL  BODY  and  the  INFUNDIBULUM  respectively, 
while  from  similar  regions  of  the  mesencephalon  are  de- 
veloped the  OPTIC  LOBES  arid  CRURA  CEREBRI.  Hand  in  hand 
with  these  changes  the  primary  cavities  (VENTRICLES)  of  the 
chambers  undergo  a  gradual  constriction,  but  throughout  all 
there  persists  at  least  a  remnant  of  the  original  tubular  cavity 
which  is  continuous  with  that  of  the  spinal  cord.  (Fig.  105.) 

The  brain  and  spinal  cord  lie,  as  we  know,  imbedded  in 
the  muscles  forming  the  dorsal  part  of  the  body  wall,  and 
are  protected  and  isolated  by  a  cartilaginous  or  bony  tube 
formed  by  the  skull  and  neural  arches  of  the  vertebrae. 
The  sole  paths  of  nervous  communication  between  the 
central  system  and  the  rest  of  the  organism  and  its  sur- 
roundings are  a  series  of  pairs  of  CRANIAL  and  SPINAL  NERVES. 
These  arise  at  fairly  regular  intervals  from  one  end  of 


COORDINATION    IN  ANIMALS 


189 


8 
*! 

II 

CLi    <£ 


II 


^  i 

if 

*   2 

cu  ,=5 

'>  3 

1* 

Q  Q 


190 


FOUNDATIONS    OF   BIOLOGY 


the  brain  and   cord  to   the   other,  and  pass  out   through 
openings  in  the  skull  and  between  or  through  the  vertebrae 

to  constitute  the  peripheral 
nervous  system.  (Fig.  106.) 
It  is  usually  considered 
that  the  primitive  segmen- 
tal  condition  of  the  Verte- 
brate body  is  well  exhibited 
in  the  arrangement  of  the 
cranial  and  spinal  nerves, 
and  that  the  origin  of  the 
cranial  nerves  from  the 
brain  affords  a  partial  in- 
dex to  the  primary  series 
of  metameres  which  ap- 
parently have  been  merged 
to  form  the  Vertebrate 
head.  Conditions  as  they 
exist  at  the  present  time 
can  perhaps  be  most  readi- 
ly understood  by  imagining 
a  simple,  ancestral,  seg- 
mented worm-like  form  in 
which  the  dorsal  neural 
tube  gives  off  a  pair  of 
nerves  to  each  segment  of 
the  body.  As  the  result  of 
a  gradual  shifting  forward 
and  a  consequent  coales- 
cence and  fusion  of  certain 
segments  near  the  anterior 
end,  there  is  brought  about 
the  delineation  of  a  head 


FIG.  106.  —  Ventral  view  of  the  nervous 
system  of  the  Frog.  Br,  second  and  third 
spinal  nerves  (brachial  plexus);  Js,  sciatic 
nerve  leading  from  the  sciatic  plexus;  O,  eye; 
Ol,  olfactory  nerve;  Op,  optic  nerve;  Sg  1-10, 
ten  ganglia  of  autonomic  system;  Spn  1, 
first  spinal  nerve;  Sp  4,  fourth  spinal  nerve; 
Vg,  Gasserian  ganglion;  Xg,  ganglion  of  10th 
cranial  nerve  (vagus).  (After  Ecker.) 


COORDINATION    IN   ANIMALS  191 

region,  with  its  brain,  battery  of  sense  organs,  and 
skull,  from  a  trunk  region  with  its  spinal  cord,  vertebral 
column,  paired  limbs,  etc.  This  naturally  involves  a  cor- 
responding shifting  and  modification  of  the  primitive  con- 
dition of  the  paired  nerves;  especially  since  the  innervation 
of  a  group  of  cells  in  normal  development  is  apparently 
rarely  changed  —  a  nerve  following  the  part  which  it  origi- 
nally supplied  through  many  of  the  transformations  and 
even  migrations  of  the  latter. 

If  this  point  of  view  is  accepted,  the  cranial  and  spinal 
nerves  are,  historically  considered,  similar  structures.  But 
the  former,  synchronously  with  the  changes  in  the  head 
region,  have  departed  somewhat  widely  from  their  ancestral 
condition  and  have  even  been  augmented  by  nerves  of  diverse 
origin.  The  spinal  nerves,  on  the  other  hand,  continue  to 
issue  from  the  cord  at  about  equal  intervals  and  in  metameric 
arrangement  as  indicated  by  muscle  segments  and  skeletal 
structures,  although  those  of  certain  regions  unite  in  the  body 
cavity  to  form  PLEXUSES  for  the  adequate  innervation  of  the 
appendages. 

From  the  standpoint  of  function  the  nerves  are  of  two 
classes,  SENSORY  and  MOTOR.  The  former  are  distributed 
mainly  to  the  skin  and  sense  organs  of  the  head,  and  are  the 
paths  over  which  excitations  (NERVOUS  IMPULSES)  due  to 
external  stimuli  are  conducted  to  the  cord  and  brain.  The 
motor  nerves,  on  the  other  hand,  are  the  media  for  distribut- 
ing impulses  from  the  central  organ  to  muscle  cells,  gland 
cells,  etc.,  and  thus  induce  the  response  of  the  organism. 

In  discussing  nerves,  it  must  be  kept  in  mind  that  a  nerve 
is  actually  a  bundle  of  nerve  fibers;  the  fibers  themselves  in 
turn  being  prolongations  of  nerve  cells,  the  cell  bodies  of 
which  are  usually  situated  in  groups  or  GANGLIA.  Moreover, 
nerve  impulses  are  not  transmitted  by  nerves  as  a  whole,  but 


192  FOUNDATIONS    OF   BIOLOGY 

by  one  component  cell  process,  the  nerve  fiber;  that  is,  by 
way  of  a  definite  cell  path  through  the  nerve.  The  same  is 
equally  true  of  the  cord  and  the  brain,  which  differ  from 
nerves  largely  in  the  circumstance  that  they  comprise  more 
cell  processes  and  also  the  cell  bodies  themselves.  In  other 
words,  the  brain  and  cord  comprise  the  elements  of  both 
ganglia  and  nerves. 

A  given  nerve  may  conduct  impulses  both  to  and  from  the 
central  organ  if  it  contains  afferent  and  efferent  cell  paths,  or 
fibers.  As  a  matter  of  fact  all  the  peripheral  nerves  primarily 
are  mixed  nerves,  because  typically  they  arise  by  two  roots 
from  the  central  organ;  the  DORSAL  ROOT  containing  only 
sensory  (afferent)  fibers  and  the  VENTRAL  ROOT  only  motor 
(efferent)  fibers.  This  condition  is  preserved  by  the  spinal 
nerves  of  higher  forms  since  each  arises  by  two  roots.  But 
some  of  the  cranial  nerves,  in  response  to  the  profound  modi- 
fications which  have  been  wrought  in  the  head  region,  have 
only  one  root,  and  so  are  either  solely  sensory,  as  those  to  the 
sense  organs,  or  only  motor,  as  those  innervating  the  muscles 
which  move  the  eye.  (Fig.  107.) 

So  far  we  have  considered  the  central  system  —  the  brain 
and  spinal  cord  —  and  its  lines  of  communication  with  the 
body  as  a  whole,  the  peripheral  system,  or  cranial  and  spinal 
nerves.  In  point  of  fact,  however,  the  peripheral  system 
gives  rise  to  an  auxiliary  series  of  ganglia  and  nerves  which 
are  charged  with  the  innervation  of  certain  of  the  internal 
organs,  particularly  the  alimentary  canal  and  arteries,  which 
are  not  directly  under  voluntary  control.  This  AUTONOMIC 
SYSTEM  in  the  higher  Vertebrates  consists  essentially  of  a 
double  nerve  chain  situated  chiefly  within  the  coelom  just 
ventral  to  the  spinal  column.  It  communicates  with  the 
central  system  by  way  of  the  sensory  roots  of  the  spinal  and 
some  of  the  cranial  nerves.  (Fig.  106.) 


COORDINATION    IN   ANIMALS 


193 


Such  in  essence  are  the  ramifications  throughout  the  body 
of  the  nervous  system  which,  although  it  arises  as  an  infolding 
of  the  ectoderm  and  therefore  is  primarily  external,  eomes  to 
be  internal  and  so  chiefly  dependent  upon  more  or  less  iso- 
lated groups  of  sensory  cells  for  the  reception  of  stimuli. 
Some  of  these,  termed  EXTERNAL  RECEPTORS,  remain  at  the 
surface  to  receive  stimuli  from  the  outer  world,  while  others, 


d.e 


C.COT* 


FIG.  107.  —  Diagram  of  a  section  of  the  spinal  cord  to  show  the  paths  of  nerve  im- 
pulses, c.c,  central  canal;  col,  collateral  fibers;  c.  corl,  cells  of  the  cortex  of  the  cerebral 
hemispheres  of  the  brain;  c.g,  smaller  cerebral  cells;  d.c,  cells  in  dorsal  part  of  gray 
matter;  d.r,  dorsal  root  of  spinal  nerve;  g,  ganglion  of  dorsal  root;  g.c,  cell  body  of 
sensory  neuron;  g.m,  gray  matter  of  cord;  M,  muscle;  m.c,  nerve  cell  in  medulla;  m.f,  fi- 
ber (axon)  of  motor  neuron ;  s,  sensory  surface ;  s.f,  fiber  of  sensory  neuron ;  spc,  spinal  cord ; 
v.c,  cells  in  ventral  part  of  gray  matter;  v.r,  ventral  root  of  spinal  nerve;  w.m,  white 
matter  of  cord.  The  arrows  indicate  the  direction  of  the  impulses.  (After  Parker  and 
Parker.) 

known  as  INTERNAL  RECEPTORS,  are  situated  within  the 
body  for  the  reception  of  stimuli  arising  there.  The  ex- 
ternal receptors  are  what  one  ordinarily  thinks  of  as  sense 

organs. 

C.   SENSE  ORGANS 

Although  among  some  of  the  Protozoa  certain  regions  of 
the  cell  are  specialized  so  that  they  are  more  sensitive  to 
one  or  another  kind  of  stimulation,  the  great  majority  show 
no  trace  of  sense  organs.  Nevertheless  all  forms,  in  common 
with  all  protoplasm,  possess  the  power  of  receiving  and  re- 


194 


FOUNDATIONS    OF   BIOLOGY 


spending  to  environmental  changes.  Thus  Paramecium  re- 
acts to  mechanical,  thermal,  chemical,  and  electrical  stimula- 
tion: the  en-tire  surface  of  the  cell  is  sensitive  to  stimuli,  and 


the  excitations  are  conducted  from 
one  part  to  another  essentially  by 
the  protoplasm  as  a  whole.  In  some 
Invertebrates,  such  as  Hydra  and 
the  Earthworm,  the  whole  surface 
of  the  body  is  still  depended  upon 
as  a  receiving  organ  for  all  kinds  of 
stimuli,  and  only  simple  sense 
cells  are  developed.  In  the  major- 
ity of  animals,  however,  although  all 
the  cells' retain  to  some  extent  their 
pristine  power  of  irritability,  envi- 
ronmental changes  exert  their  influ- 
ence chiefly  upon  complex  receptors, 
which  are  specialized  to  respond 
most  readily  to  particular  forms  of 
energy.  The  energy,  for  example  of 
heat  or  light,  is  transformed  by  ap- 
propriate mechanisms  into  the  ener- 
gy of  a  NERVE  IMPULSE,  and  accord- 
ingly the  sense  organs  constitute 
the  outposts  of  the  nervous  system. 
Since  we  necessarily  gain  our  knowledge  of  the  outside  world 
solely  through  the  data  afforded  by  our  sense  organs,  it  fol- 
lows that  we  judge  the  capacity  of  the  sense  organs  of  other 
animals  merely  by  analogy  with  our  own.  This  is  a  safe  pro- 
cedure only  in  the  case  of  sense  organs  which  more  or  less 
correspond  in  structure  to  those  which  we  possess.  In  the 
Crayfish,  for  example,  we  find  complex  sense  organs  which, 
without  doubt,  are  eyes,  and  others  which  are  ears,  or  at  least 


ABC 

FIG.  108.  —  Diagram  of  stages 
in  the  differentiation  of  sense 
cells.  A,  primitive  sensory  neu- 
ron of  Hydra-like  animals ;  B,  sen- 
sory neuron  of  a  Mollusc;  C, 
primary  sensory  neuron  of  a 
Vertebrate.  In  each  case  the 
sensory  surface  is  represented 
below,  and  therefore  the  nerve 
impulse  passes  upward.  (After 
G.  H.  Parker.) 


COORDINATION    IN    ANIMALS  195 

perform  one  of  the  functions  of  our  ears,  equilibration;  while 
some  of  the  head  appendages  are  particularly  adapted  to 
receive  sensations  of  touch.  The  senses  of  smell  and  taste  are 
also  probably  present,  but  here  we  are  on  less  certain  ground. 
It  is  possible,  perhaps  probable,  that  environmental  changes 
which  are  without  effect  on  the  sense  organs  of  the  human 
body,  and  so  play  no  recognizable  part  in  the  'world'  of 
Man,  may  stimulate  receptors  in  lower  organisms. 

The  simplest  form  of  sense  organ  in  Vertebrates  is  a  single 
epithelial  cell  for  the  reception  of  stimuli,  connected  with  a 
nerve  fiber  for  the  conduction  of  the  nerve  impulse  to  a  sen- 
sory center.  Usually,  however,  many  associated  cells  are 
arranged  to  respond  and  are  aided  by  accessory  structures  for 
intensifying  the  stimulus,  protection,  etc  ,  so  that  the  whole 
forms  a  highly  complex  sense  organ.  (Figs.  108C,  112.) 

1.   Cutaneous  Senses 

Confining  our  attention  to  the  Vertebrates  we  find  that 
practically  the  entire  surface  of  the  body  constitutes  a  sense 
organ,  because  the  skin  is  permeated  with  a  network  of  sen- 
sory nerves.  Certain  regions  are  supplied  with  special  tactile 
organs,  which  may  take  the  form  of  a  regular  system  of  sense 
organs,  such  as  the  LATERAL  LINE  ORGANS  of  Fishes  and  Am- 
phibians, or  of  groups  of  TACTILE  CORPUSCLES  as  in  Man.  In 
addition  to  pressure  receptors,  the  whole  surface  of  the  human 
body  is  provided  with  PAIN,  HEAT,  and  COLD  SENSE  SPOTS. 

2.   Sense  of  Taste 

In  the  higher  Vertebrates  the  sense  of  taste  is  restricted 
to  the  cavity  of  the  mouth,  particularly  to  the  tongue,  where 
special  receptors  known  as  TASTE  BUDS  are  in  communication 
with' the  brain  by  two  of  the  cranial  nerves;  but  in  some  Fishes 
they  are  scattered  quite  generally,  so  that  the  whole  body 
surface  is  sensitive  to  such  qualities  as  sweet,  sour,  and  salt. 


196  FOUNDATIONS   OF   BIOLOGY 

3.   Sense  of  Smell 

The  special  sense  organs  of  smell,  or  OLFACTORY  BUDS, 
reside  in  the  membrane  which  lines  a  pair  of  invaginations  of 
the  anterior  end  of  the  head,  termed  OLFACTORY  POUCHES. 

The  buds  are  in  communication  with  the  brain  by  the  olfac- 
tory, or  first  pair  of  cranial  nerves.  The  pouches  constitute 
relatively  simple  sacs  in  the  lower  Vertebrates,  but  in  the  air- 
breathing  forms,  and  especially  in  the  Mammals,  the  walls  of 
the  pouches  are  thrown  into  folds,  ridges,  and  secondary 
pouches.  This  is  necessitated  by  the  concentration  of  the 
olfactory  surface  to  the  air  passages  of  the  nose  which  lead 
to  the  lungs.  On  the  other  hand,  in  Man  the  olfactory  appa- 
ratus has  fallen  somewhat  from  the  complexity  which  it  attains 
in  the  lower  Mammals,  as  is  attested  not  only  by  its  structure 
in  the  adult  but  also  by  transient  remnants  in  the  human  em- 
bryo. 

4.    The  Ear 

The  ears,  or  organs  of  hearing  and  equilibration,  arise  as 
paired  depressions  of  the  ectoderm  of  the  head,  which,  in 
all  Vertebrates  above  the  lower  Fishes,  lose  their  connection 
with  the  exterior  and  form  the  so-called  INNER  EAR,  or  LABY- 
RINTH. This  becomes  divided  into  two  chief  parts,  the  SAC- 
CULUS  and  the  UTRICULUS  from  which  are  developed  three 
SEMICIRCULAR  CANALS,  one  in  each  plane  of  space.  The  sac- 
culus  is  largely  devoted  to  the  reception  of  vibrations  of  the 
surrounding  medium,  that  is  to  hearing  in  the  usual  sense  of 
the  word.  Accordingly  the  sacculus  becomes  progressively 
differentiated  as  we  ascend  the  Vertebrate  scale  —  a  complex 
derivative  in  the  mammalian  ear  being  the  COCHLEA.  On  the 
other  hand,  the  utriculus  and  the  semicircular  canals  provide 
for  sensations  of  loss  of  equilibrium,  or  orientation  of  the 
body  in  space,  and  show  far  less  change.  It  is  probable 


COORDINATION   IN   ANIMALS 


197 


that  equilibration  is  the  chief  function  of  the  entire  labyrinth 
in  Fishes,  as  it  is  of  the  so-called  auditory  organs  of  many 
Invertebrates,  such  as  the  Crayfish.  With  the  progres- 
sive specialization  of  the  labyrinth,  the  essential  sensory 
cells,  which  are  in  communication  with  the  brain  by  the 
eighth,  or  AUDITORY  NERVE,  become 
limited  to  a  few  definite  areas. 
These  sensory  cells  are  provided 
with  auditory  hairs  which  project 
into  the  cavity  of  the  labyrinth  and 
so  are  stimulated  by  movements  of 
the  fluid  which  fills  it.  (Fig.  109.) 

The  ears  of  Fishes  lie  immediately 
below  the  skull  roof,  where  they  are 
readily  accessible  to  vibrations 
transmitted  by  the  water.  But 
with  the  substitution  of  air  for  water 
as  the  surrounding  medium,  there 
arises  the  necessity  of  a  more  deli- 
cate method  for  conducting  and 
also  for  collecting  and  augmenting 
the  sound  waves.  The  result  is 
that,  in  ascending  the  Vertebrate 
series,  we  find  the  ear  proper  receding  farther  and  farther 
below  the  surface. 

Soon,  between  the  inner  ear  and  the  surface  of  the  head,  a 
simple  resonating  chamber  is  added  which  is  provided  with  a 
vibrating  TYMPANIC  membrane,  or  EAR  DRUM,  situated  just 
under  the  skin.  Then  this  is  improved  by  the  development 
of  a  bony  transmitting  mechanism  between  the  tympanic 
membrane  and  the  inner  ear.  This  consists  of  a  single  bone 
until  we  reach  the  Mammals,  when  two  more  bones  are  added 
by  being  diverted  from  their  earlier  function  of  articulating 


Fio.  109.  —  Semidiagram- 
matic  figure  of  the  left  mem- 
branous labyrinth  of  a  lower 
Vertebrate  to  show  the  sacculus 
(s,  I),  utriculus  (u,  rec),  and  the 
three  semicircular  canals  (aa,  ca; 
ae,  ce,  and  ap,  cp).  I,  lagena,  a 
derivative  of  the  sacculus  which 
becomes  the  cochlea  in  higher 
Vertebrates;  cus,  utriculo-saccu- 
lar  canal;  de,  se,  endolymphatic 
duct  and  sac ;  ass,  sp,  ss,  utricular 
sinuses.  (After  Wiedersheim.) 


198 


FOUNDATIONS    OF    BIOLOGY 


the  jaws  with  the  skull!    Finally,  the  resonating  (tympanic) 
chamber  recedes  farther  below  the  surface  and  becomes  the 


FIG.  110.  —  Front  view  of  the  human  organ  of  hearing,  right  side,  a,  pinna  of  outer 
ear;  b,  bone  of  skull;  c,  d,  I,  transmitting  mechanism  of  three  bones — 'malleus,  incus, 
and  stapes;  e,  one  of  the  three  semicircular  canals;  g,  vestibule;  h,  auditory  nerve;  i, 
cochlea;  j,  Eustachian  tube  leading  to  the  throat;  k,  tympanic  chamber  or  middle  ear; 
/,  stapes;  m,  tympanic  membrane;  n,»  external  auditory  passage,  or  outer  ear;  o, 
cartilage. 

MIDDLE  EAR  to  which  sound  waves  are  conducted  through  a 
tubular  passage,  the  OUTER  EAR.  In  some  forms,  as  in  Man, 
there  is  an  external  funnel-like  collecting  appendage,  the 
PINNA.  (Fig.  110.) 

5.    The  Eye 

The  organs  of  sight  are  the  most  complex  sense  organs  of 
animals  and  reach  a  very  high  degree  of  specialization  even 
in  some  of  the  Invertebrate  forms.  Among  the  latter  the 
essential  sensory  element  (RETINA)  of  the  eye  usually  arises 
by  the  invagination  of  a  limited  area  of  ectoderm,  the  cells 
of  which  become  differentiated  for  receiving  the  photic 
stimuli  that  produce  impulses  to  be  transmitted  to  the  central 
nervous  system.  Among  Vertebrates  the  sensory  cells  are  also 
of  ectodermic  origin,  but  only  secondarily  so,  since  the  OPTIC 


COORDINATION    IN   ANIMALS  199 

VESICLES  arise  as  lateral  outpocketings  directly  from  the  fore- 
brain.  (Fig.  111.) 

A  retina  alone  such  as  exists  in  some  of  the  lower  Inverte- 
brates can  afford  no  visual  sensations  other  than  light  and 
darkness,  and  perhaps  in  some  cases  the  ability  to  distinguish 
light  of  one  color  from  that  of  another.  In  order  that  not 
merely  degrees  of  the  intensity  of  light  may  be  perceived,  but 
that  objects  may  be  seen,  many  of  the  higher  Invertebrates 
have  developed  various  kinds  of  complicated  apparatus  for 
bringing  the  rays  from  a  given  point  to  a  focus  at  one  point 
on  the  retina,  culminating  on  the  one  hand  in  the  mosaic 
vision  of  the  Arthropods,  and  on  the  other  hand  in  the  camera 
eye  of  the  Cuttlefish.  In  the  latter  case  the  mechanism  is 
quite  similar  to  that  found  in  the  Vertebrates,  but  since  it 
occurs  in  the  group  of  Molluscs  which  cannot  be  considered 
in  the  direct  evolutionary  line  of  the  Vertebrates,  it  affords 
an  example  of  similar  responses  of  different  organisms  to 
similar  needs  giving  rise  to  analogous  structures.  (Fig.  112.) 

In  the  development  of  the  Vertebrate  eye,  the  hollow  out- 
growth or  optic  vesicle  (one  of  which  arises  from  either  side 
of  the  diencephalon)  gradually  extends  toward  the  outer  sur- 
face of  the  head,  where  it  becomes  associated  with  an  in- 
pocketing  of  the  ectoderm.  The  latter  gradually  becomes 
separated  from  the  surface  ectoderm  as  a  sac,  the  very  thick 
walls  of  which  almost  completely  obliterate  its  cavity.  This 
sac  is  destined  to  become  the  LENS,  and  as  it  enlarges  it  comes 
in  contact  with  the  optic  vesicle,  which  now  is  connected 
with  the  point  of  origin  from  the  diencephalon  by  a  narrow 
isthmus  (OPTIC  STALK).  Apparently  under  the  influence  of 
the  developing  lens,  the  optic  vesicle  is  invaginated  and  there- 
by transformed  from  a  single-layered  structure  into  a  double- 
layered  cup  (OPTIC  CUP).  These  two  layers  form  the  retina, 
the  inner  layer  becoming  differentiated  into  the  essential 


200 


FOUNDATIONS   OF  BIOLOGY 


a — £ 


h  — 


— c 


I) 


FIG.  111.  — Diagrams  illustrating  the  method  of  formation  of  the  eye  of  an 
Invertebrate  (A)  and  a  Vertebrate  (B,  C,  D,  E,  —  successive  stages).  Note  that 
the  opposite  surface  of  the  retinal  cells  is  exposed  to  the  light  rays  in  the  Verte- 
brates as  compared  with  the  Invertebrate  Eye.  a,  ectoderm;  b,  retinal  area;  c, 
future  position  of  optic  nerve;  d,  cavity  of  the  diencephalon;  e,  optic  vesicle;  /, 
stalk  of  optic  vesicle  later  replaced  by  the  optic  nerve;  g,  vitreous  chamber  with- 
in optic  cup;  h,  developing  lens. 


COORDINATION    IN   ANIMALS 


201 


visual  elements  (RODS  and  CONES)  of  the  eye,  while  the  outer 
supplies  the  PIGMENTED  LAYER.  The  nerve  cells  of  the  retina 
develop  fibers  which  proceed  to  the  brain  through  the  path 


FIG.  112.  — The  Vertebrate  eye  (human).  A,  vertical  section  of  the  eye  in  situ. 
B,  horizontal  section  to  show  relation  of  optic  nerve  to  fovea  centralis  through  which 
the  optical  axis  passes,  a,  eyelash;  b,  lid;  c,  bony  orbit;  d,  superior  rectus,  one  of  the 
six  muscles  which  revolve  the  eyeball;  e,  muscle  to  upper  lid;  /,  optic  nerve  (bundles 
of  fibers  cut  obliquely);  g,  inferior  rectus  muscle  of  eyeball;  h,  anterior  chamber  filled 
with  aqueous  humor;  i,  pupil,  opening  to  posterior  chamber,  also  filled  with  aqueous 
humor,  between  iris  and  lens;  j,  conjunctiva,  a  transparent  membrane,  continuous  with 
the  lining  of  the  eyelid;  k,  cornea;  I,  iris;  m,  lens;  n,  suspensory  ligament  of  lens; 
o,  retina;  p,  choroid  coat;  q,  sclerotic  coat;  r,  muscles  to  ligament  suspending  lens; 
s,  vitreous  chamber  containing  vitreous  humor;  t,  point  of  entrance  of  optic  nerve 
('blind  spot');  u,  fatty  connective  tissue;  x,  fovea  centralis  at  posterior  end  of  axis 
of  eyeball. 

occupied  by  the  optic  stalk  and  so  give  rise  to  the  OPTIC 
NERVE. 

To  the  optic  cup  and  lens,  the  former  indirectly  and  the 
latter  directly  of  ectodermal  origin,  other  portions  largely  of 
mesodermal  origin  are  added  —  e.  g.,  the  CORNEA,  CHOROID 


202  FOUNDATIONS    OF   BIOLOGY 

and  SCLEROTIC  COATS,  the  IRIS,  and  the  VITREOUS  HUMOR  - 
all  of  which  contribute  to  the  make-up  of  the  eye-ball.  The 
eye  of  Vertebrates  is  an  optical  apparatus  which  may  be  com- 
pared  roughly  with  a  camera.  Light  waves  which  pass 
through  an  outer  transparent  protective  coating  and  an 
opening  (PUPIL)  in  a  regulating  diaphragm  (iris)  reach  the 
lens  and  are  brought  to  a  focus  on  the  retina.  The  sensory 
stimulation  thus  brought  about  is  transmitted  by  the  optic 
or  second  cranial  nerve  to  the  brain. 

A  broad  survey  of  the  sense  organs  of  Vertebrates  impresses 
one  with  the  fact  that,  taken  by  and  large,  the  improvements, 
though  considerable,  are  not  so  marked  as  one  might  expect 
when  the  great  development  of  the  nervous,  system,  and  the 
brain  in  particular,  is  considered.  And  so  we  must  look 
chiefly  to  the  cumulative  influence  of  the  sensory  stimuli 
themselves  for  the  underlying  factor  in  the  development  of 
the  brain  during  its  long  evolutionary  history  —  the  brain, 
in  turn,  being  enabled  to  make  more  out  of  the  same  stimuli 
and  create  in  Man  the  higher  mental  life  with  all  that  it  im- 
plies. 


CHAPTER  XV 
REPRODUCTION   IN  ANIMALS 

So  careful  of  the  type  .  .  . 

So  careless  of  the  single  life.  —  Tennyson. 

IN  addition  to  the  organs  devoted  to  the  life  of  the  individ- 
ual animal,  the  Vertebrates  in  common  with  all  forms  of  life 
necessarily  are  provided  with  means  for  the  continuation  of 
the  life  of  the  race.  Reproduction,  it  will  be  recalled,  is, 
in  the  last  analysis,  division;  the  setting  free  by  the  organism 
of  cells  with  the  power  of  going  through  a  complex  series  of 
changes,  involving  cell  division  and  differentiation,  by  which 
the  relatively  simple  germ  cell  becomes  transformed  into  the 
obviously  complex  individual,  similar  to  the  parent.  In  most 
plants  and  animals  this  process  is  complicated  at  the  start  by 
the  fusion  of  two  germ  cells,  the  male  and  female  gametes, 
to  form  the  fertilized  egg,  or  zygote.  Disregarding  for  the 
time  being  the  ultimate  origin  of  the  germ  cells  in  the  body, 
we  find  in  the  Metazoa  special  organs  in  which  the  germ  cells 
reside  and  undergo  changes  preparatory  to  their  liberation. 
Such  reproductive  organs,  or  oojiAps,  ordinarily  contain  germ 
cells  of  one  kind,  and  accordingly  are  either  OVARIES  (egg- 
producing  organs)  or  TESTES  (sperm-producing  organs) . 

In  many  of  the  simpler  animals,  the  gonads  are  merely 
temporary  structures  which  appear  during  certain  seasons  of 
the  year  when  conditions  favor  sexual  reproduction.  Fre- 
quently also  the  same  individual  produces  both  eggs  and 
sperm,  in  wrhich  case  the  sexuality  of  the  germ  cells  is  not 
reflected  back,  so  to  speak,  to  the  organism  as  a  whole,  which 

203 


204  FOUNDATIONS   OF   BIOLOGY 

accordingly  is  known  as  a  HERMAPHRODITE.  Such  is  the 
condition  in  Hydra,  where  the  testes  appear  as  small  swellings 
in  the  ectoderm  a  little  below  the  circle  of  tentacles;  and  the 
ovary,  which  is  usually  single,  is  a  somewhat  larger  projection 
near  the  opposite  end  of  the  animal.  Both  the  testis  and  the 
ovary  at  first  appear  to  be  a  heap  of  ectoderm  cells,  which  in 
one  case  gives  rise  to  many  sperm  and  in  the  other  to  a  single 
egg.  The  mature  sperm  are  set  free  from  the  testis  and  swim 
about  in  the  water.  Sooner  or  later  one  enters  the  now  rup- 
tured covering  of  the  ovary  and  fuses  with  the  egg.  With  the 
conclusion  of  fertilization  the  zygote  begins  to  divide  and 
forms  an  embryo,  which  at  an  early  stage  becomes  detached 
from  the  parent.  Thus  in  Hydra  there  is  no  complicated 
apparatus  for  sexual  reproduction;  merely  now  and  again 
the  temporary  development  of  the  primary  sex  organs, 
ovaries  and  testes.  (Fig.  64.) 

The  complex  bodies  of  most  animals,  however,  demand 
more  or  less  permanent  gonads  as  well  as  means  for  trans- 
ferring the  gametes  directly  or  indirectly  to  the  exterior. 
This  is  brought  about  by  the  fact  that  in  coelomate  animals 
the  gonads  come  to  lie,  not  on  the  outside  of  the  body,  but 
within  the  coelom.  In  the  Earthworm,  which  also  is  her- 
maphroditic, the  testes  and  ovaries  are  permanent  organs 
attached  to  the  partitions  between  certain  somites.  The 
sexual  products  are  set  free  in  the  coelom,  where  they  are 
taken  up  by  SPERM  DUCTS  and  OVIDUCTS  and  carried  to  the 
outside.  Although  each  Earthworm  possesses  both  male 
and  female  reproductive  organs,  two  worms  copulate  and 
exchange  sperm  which  are  stored  in  the  respective  seminal 
receptacles.  Later,  when  the  eggs  pass  to  the  exterior,  the 
'foreign'  sperm  are  shed  on  them.  Thus  cross-fertilization  is 
insured  in  this  hermaphroditic  form.  In  the  Crayfish  the 
sexes  are  represented  by  separate  individuals,  and  the  appen- 


REPRODUCTION    IN   ANIMALS 


205 


dages  of  the  first  and  second  abdominal  segments  of  the 
male  are  modified  into  copulatory  organs  for  the  transfer  of 
the  sperm  to  the  body  of  the  female,  where  they  are  retained 


FIG.  113.  —  Diagrammatic  section  of  the  human  uterus  with  developing  embryo. 
The  embryo  (h)  is  suspended  in  a  fluid-filled  cavity  (c)  surrounded  by  the  foetal  mem- 
branes (e)  and  by  tissue  (/)  from  the  uterus  itself.  The  sole  path  of  communication 
between  embryo  and  mother  is  by  blood  in  vessels  passing  up  through  the  umbilical 
cord  (i),  spreading  out  into  capillaries  in  the  placenta  (6)  and  there  coming  into  close 
relations  with  the  maternal  blood  supply.  The  openings  of  the  oviducts  (d)  into  the 
uterus  become  closed  during  the  development  of  the  embryo,  a,  dorsal  wall  of  uterus; 
b,  placenta;  c,  fluid-filled  cavity  of  amnion;  d,  openings  of  oviducts  (Fallopian  tubes); 
e,  foetal  membranes;  /,  uterine  tissue;  g,  uterine  cavity;  h,  embryo;  i,  umbilical  cord. 

until  egg-laying.  In  most  terrestrial  Vertebrates,  including 
Man,  fertilization  occurs  while  the  eggs  are  still  within  the 
oviducts,  the  copulatory  organs  transferring  the  sperm 
directly  to  the  terminal  portion  of  the  ducts  from  which  they 
make  their  way  up  to  meet  the  descending  eggs.  (Figs.  67, 
71,  72,  86.) 


206  FOUNDATIONS   OF   BIOLOGY 

When  fertilization  occurs  within  the  body,  the  egg  may 
soon  pass  to  the  exterior,  usually  after  being  wrapped  up 
in  nutritive  and  protective  coats  secreted  about  it  during  its 
passage  down  the  oviduct.  Or,  as  is  the  case  sporadically 
among  lower  forms  and  the  rule  among  the  highest  Verte- 
brates, the  Mammals,  the  egg  on  reaching  the  lower  part 
of  the  oviduct  may  become  attached  to  the  wall  of  an  en- 
largement of  the  oviduct,  or  of  a  chamber  formed  by  the 
union  of  the  two  oviducts,  called  the  UTERUS.  Here  the 
embryo  derives  nourishment  from  the  maternal  blood  sup- 
ply, and  proceeds  far  along  in  development  before  it  is  ex- 
pelled to  the  exterior,  or  born.  (Fig.  113.) 

Thus,  except  in  the  simplest  animals,  there  is  a  special 
REPRODUCTIVE  SYSTEM;  a  series  of  organs  connected  with 
the  reproductive  function.  But  it  must  be  emphasized 
that  the  essential  organs  are  the  gonads  themselves  and  all 
the  rest  are  accessory.  Furthermore,  in  relation  to  the  sexual 
differentiation  of  male  and  female  individuals,  many  so-called 
SECONDARY  SEXUAL  CHARACTERS  arise  which  are  not  directly 
connected  with  the  reproductive  organs,  but  nevertheless 
depend  very  largely  for  their  development  upon  hormones 
liberated  by  the  gonads.  For  .example,  early  castration  of 
the  Stag  inhibits  the  growth  of  a  distinctive  male  secondary 
sexual  character,  the  antlers;  while  if  performed  later  when 
the  antlers  are  full  grown,  they  are  shed  and  abnormal  ones 
take  their  place.  Similarly,  the  development  and  functioning 
of  the  mammary  glands  during  pregnancy  in  the  human  fe- 
male is  induced  by  hormones  produced,  not  by  the  ovary  it- 
self, but  by  its  product,  the  developing  embryo  within  the 
uterus.  Here  at  least  two  hormones  are  involved;  one 
directly  stimulates  the  development  of  the  glands,  while 
another  inhibits  their  active  functioning  until  it  is  removed 
by  the  birth  of  the  offspring.  (See  p.  181.) 


REPRODUCTION    IN   ANIMALS  207 

Throughout  all  the  chief  Vertebrate  groups  the  sexes  are 
distinct,  although  in  rare  instances  abnormal  hermaphro- 
ditic individuals  occur.  The  definitive  primordial  germ  cells 
first  appear  as  localized  areas  of  the  coelomic  epithelium,  on 
either  side  of  the  vertebral  column.  As  the  germ  cells  develop 
they  become  associated  with  connective  tissue,  blood  vessels, 
and  nerves  and  form  the  paired  gonads.  In  the  most  primi- 
tive Vertebrates  a  condition  more  simple  than  in  the  Earth- 
worm is  found,  for  both  male  and  female  germ  cells  when  ripe 
merely  break  out  of  the  gonads  and  find  their  way  to  the  ex- 
terior by  a  pair  of  minute  ABDOMINAL  PORES.  In  -higher 
forms,  however,  the  labor  of  conducting  the  products  out  of 
the  body  is  foisted  upon  the  urinary  system,  as  was 
suggested  when  that  system  was  under  discussion.  We 
now  turn  to  a  statement  of  the  structural  inter-relations 
of  these  two  systems  to  form  the  UROGENITAL  SYSTEM. 

It  has  been  pointed  out  that  the  nephridia,  which  combine 
to  form  the  kidneys,  in  some  of  the  lower  Vertebrates  retain 
their  funnel-like  openings  into  the  coelom  and  therefore  afford 
a  direct  exit  for  waste  material  in  the  coelomic  fluid.  It  is 
some  of  these  nephridia  which  are  employed  in  the  lower 
Fishes  for  the  transfer  of  the  germ  cells  to  the  outside.  The 
testes  of  the  male,  which  lie  close  to  the  kidneys,  become  con- 
nected with  the  nephridia  (mesonephros)  by  a  series  of  short 
delicate  tubes.  Through  these  tubes  the  SPERMATIC  FLUID, 
containing  the  sperm  from  the  testes,  is  transferred  to  the 
nephridia  and  by  them  to  the  kidney  (mesonephric)  ducts 
and  so  to  the  exterior  with  the  urinary  waste.  In  this  way, 
during  the  period  of  sexual  activity  of  the  male,  the  kidney 
tubules  satisfactorily  perform  two  functions,  and  the  mesone- 
phric ducts  become  UROGENITAL  CANALS.  (Fig.  97,  C.) 

Turning  to  the  female,  we  find  that  the  ovaries,  which  are 
situated  in  about  the  same  position  with  relation  to  the  kid- 


208  FOUNDATIONS    OF   BIOLOGY 

neys  as  the  testes  in  the  male,  do  not  enter  into  communica- 
tion with  a  set  of  nephridia  of  the  kidneys  (mesonephros) ; 
probably  because  the  eggs  are  too  large  to  pass  through  the 
tubules.  Instead,  what  appears  to  be  the  coelomic  opening, 
or  NEPHROSTOME,  of  a  single  nephridium  on  either  side  (which 
fails,  so  to  speak,  to  enter  the  kidney  complex)  enlarges  and 
becomes  the  funnel  which  connects  up  with  a  new  duct  open- 
ing into  the  cloaca.  Thus  there  arises  from  the  female  urinary 
system  a  pair  of  entirely  distinct  OVIDUCTS.  An  egg,  liberated 
from  the  ovary  into  the  coelom,  finds  its  way  into  one  of  the 
oviducts  and  descends  directly  to  the  outside,  or  into  an  en- 
largement (uterus)  of  the  terminal  portion  of  the  duct  where 
development  proceeds  until  birth  occurs.  (Fig.  97,  D.) 

The  female  reproductive  system,  though  derived  from  the 
mesonephric  system,  has  become  entirely  independent  of  it. 
Accordingly  the  disappearance  of  the  mesonephros  and  duct 
in  higher  Vertebrates,  when  it  is  replaced  by  the  metanephros 
and  the  ureter  as  the  functional  urinary  system,  has  little 
effect  on  the  female  reproductive  system.  As  a  matter  of 
fact  the  abandoned  mesonephros  and  duct  degenerate  and 
disappear  in  the  female,  while  in  the  male  the  mesonephric 
duct  remains  and  becomes  completely  appropriated  by  the 
reproductive  system.  The  sperm  now  pass  directly  into  the 
former  mesonephric  duct,  which  thereby  becomes  solely  a 
sperm  duct.  Such  is  the  historical  origin  of  the  foundations 
of  the  reproductive  system  as  it  occurs  in  the  Reptiles,  Birds, 
and  Mammals.  Naturally  each  of  these  groups,  building  on 
this  foundation,  has  developed  modifications  and  additions 
demanded  by  its  special  lines  of  evolution;  (Fig.  97,  E,  F.) 


CHAPTER  XVI 
ORIGIN  OF  THE  INDIVIDUAL 

Owing  to  the  imperfection  of  language  the  offspring  is  termed 
a  new  animal,  but  is  in  truth  a  branch  or  elongation  of  the 
parent.  —  Erasmus  Darwin,  1794. 

A  GENERAL  background  of  biological  facts  and  principles 
has  now  been  established  and  we  are  therefore  in  a  position 
to  take  up  from  an  advantageous  viewpoint  some  of  the 
broad  questions  relating  to  the  origin  of  life  and  the  origin 
of  species,  that  is  the  origin  of  individuals  since  life  and 
species  are  merely  concepts,  and  individuals  are  the  realities 
in  living  nature. 

A.   ORIGIN  OF  LIFE 

It  must  seem  strange  to  the  reader,  with  some  of  the  com- 
plexities of  organisms  before  him,  that  the  best  minds  up  to 
the  seventeenth  century  saw  nothing  more  incongruous  in 
the  spontaneous  origin  of  plants  and  animals  of  all  kinds  from 
mud  and  decaying  matter,  than  does  the  boy  of  to-day  who 
believes  that  horse  hairs  soaked  in  water  are  transformed  into 
worms.  As  a  matter  of  fact,  we  find  that  even  Aristotle,  who 
laid  such  broad  foundations  for  the  science  and  philosophy  of 
the  organism,  believed  that  certain  of  the  Vertebrates,  such 
as  Eels,  arose  spontaneously. 

Naturally,  with  the  increase  of  knowledge,  the  idea  of 
SPONTANEOUS  GENERATION  was  gradually  restricted  more  and 
more  to  the  lower  forms.  It  remained,  however,  for  Redi 
during  the  latter  half  of  the  seventeenth  century  to  question 
seriously  the  general  proposition  and  to  substitute  direct 

209 


210  FOUNDATIONS    OF   BIOLOGY 

experimentation  for  academic  discussion  and  hearsay.  By 
the  simple  expedient  of  protecting  decaying  meat  from  con- 
tamination by  flies,  he  demonstrated  that  these  insects  are 
not  developed  from  the  flesh  and  that  the  apparent  trans- 
formation of  meat  into  maggots  is  due  solely  to  the  develop- 
ment of  the  eggs  deposited  thereon  by  flies. 

But  the  time-honored  doctrine  of  spontaneous  generation 
was  not  overthrown  by  this  experiment  nor  the  long  series 
which  Redi  made.  The  presence  of  parasites  within  certain 
internal  organs  of  the  higher  animals ,  as  in  the  brains  of 
sheep,  baffled  Redi  himself.  Also,  the  improvements  of  the 
microscope  revealed  an  unknown  microcosm  whose  origin 
seemed  plausibly  explained  as  spontaneous.  BIOGENESIS,  or 
all  life  from  preexisting  life,  was  placed  on  a  secure  foundation 
only  within  the  past  half-century  by  the  working  out  of  the 
remarkably  complex  life  histories  of  internal  parasites,  which 
showed  that  they  all  arise  from  parents  like  themselves, 
and  by  the  classical  demonstrations  of  Pasteur  and  others 
that  microorganisms  are  not  the  result,  but  the  cause  of  de- 
cay. The  latter  fact  is  at  the  basis  of,  and  is  attested  by,  the 
methods  now  universally  used  in  food  preservation  and 
aseptic  surgery  —  to  mention  but  two  instances. 

At  the  present  time,  we  may  consider  it  as  established  that 
all  known  forms  of  life  arise  from  preexisting  life  by  reproduc- 
tion. But  if  we  accept  the  testimony  of  astronomer  and 
geologist,  the  Earth  was  at  one  time  in  a  condition  in  which 
life  as  we  know  it  could  not  exist,  and  so  we  are  face  to  face 
with  the  problem  of  how  it  came  to  be  established  on  the 
Earth  in  the  past  —  the  remote  past,  since  the  geological 
record  affords  convincing  proof  that  life  has  existed  continu- 
ously on  the  Earth  for  some  hundreds  of  millions  of  years. 

Unless  one  is  willing  to  ascribe  life's  origin  to  SPECIAL 
CREATION  —  which  at  once  removes  it  from  the  sphere  of 


ORIGIN    OF   THE    INDIVIDUAL  211 

science  and  so  beyond  the  present  discussion  —  or  to  AN- 
OTHER PLANET  from  whence  it  was  transferred  through 
space  to  the  Earth  —  which  removes  it  to  a  "  conveniently 
inaccessible  place  where  its  solution  is  impossible"  -there 
remains  but  one  alternative:  life  arose  through  the  gradual 
evolutionary  complexification  of  matter  when,  ages  ago, 
Earth  conditions  became  favorable.  Such  living  matter  must 
have  been  relatively  simple  compared  with  protoplasm  as 
we  know  it  today;  so  simple,  in  fact,  that  we  would  not 
recognize  it  as  such,  because  protoplasm  as  we  see  it  even  in 
the  simplest  organisms  has  had  a  long  evolutionary  history. 
Of  course  it  is  not,  a  priori,  impossible  that  such  simple  life 
is  even  at  the  present  time  arising  spontaneously  under  spe- 
cial environmental  conditions,  perhaps  in  the  ocean  depths, 
but  is  unable  to  come  to  fruition  in  competition  with  existing 
protoplasm  of  ancient  pedigree  and  evolutionary  specializa- 
tion. 

However  that  may  be,  during  the  past  quarter  century 
some  biologists  have  now  and  then  thought  they  were  on  the 
verge  of  artificially  creating  life  in  the  test  tube,  only  to  leave 
the  problem,  like  the  alchemists  of  old,  with  more  respect  for 
the  complexities  of  its  organization  and  the  "  enormous  gap 
which  separates  even  the  simplest  forms  of  life  from  the  in- 
organic world."  And  so  we  may  more  profitably  turn  to  a 
consideration  of  the  present-day  manifestations  of  life  in  the 
reproduction  of  organisms,  and  dismiss  the  insolvable  prob- 
lem of  the  origin  of  life  on  the  Earth  with  the  conservative 
statement  penned  over  forty  years  ago  by  Huxley: 

"  Looking  back  through  the  prodigious  vista  of  the  past,  I 
find  no  record  of  the  commencement  of  life,  and  therefore  I 
am  devoid  of  any  means  of  forming  a  definite  conclusion  as 
to  the  conditions  of  its  appearance.  Belief,  in  the  scientific 
sense  of  the  word,  is  a  serious  matter,  and  needs  strong 


212  FOUNDATIONS    OF   BIOLOGY 

foundations.  To  say,  therefore,  in  the  admitted  absence  of 
evidence,  that  I  have  any  belief  as  to  the  mode  in  which  exist- 
ing forms  of  life  have  originated,  would  be  using  words  in  a 
wrong  sense.  But  expectation  is  permissible  where  belief  is 
not;  and  if  it  were  given  to  me  to  look  beyond  the  abyss  of 
geologically  recorded  time  to  the  still  more  remote  period 
when  the  Earth  was  passing  through  physical  and  chemical 
conditions,  which  it  can  no  more  see  again  than  a  man  can 
recall  his  infancy,  I  should  expect  to  be  a  witness  of  the 
evolution  of  living  protoplasm  from  not  living  matter.  .  .  . 
That  is  the  expectation  to  which  analogical  reasoning  leads 
me;  but  I  beg  you  once  more  to  recollect  that  I  have  no  right 
to  call  my  opinion  anything  but  an  act  of  philosophical  faith." 
Since  so  far  as  is  known  all  life  now  arises  from  preexisting 
life  and  has  done  so  since  matter  first  assumed  the  living  state, 
it  apparently  follows  that  the  stream  of  life  is  continuous 
from  the  remote  geological  past  to  the  present  and  that  all 
organisms  of  to-day  have  an  ancient  pedigree.  It  is  to  the 
establishment  of  this  as  the  reasonable  conclusion  from  the 
data  accumulated  during  recent  years,  that  from  now  on  our 
attention  is  somewhat  more  particularly  directed;  and  ac- 
cordingly it  is  necessary  first  of  all  to  consider  in  some  detail 
the  genetic  connection  of  present-day  forms  as  exhibited  in 
reproduction. 

B.   REPRODUCTION 

The  power  of  producing  new  individuals  specifically  similar 
to  the  parent  is,  as  has  been  seen,  one  of  the  most  important 
characteristics  of  living  in  contrast  with  lifeless  matter,  and 
is  exhibited  in  its  simplest  form  in  the  unicellular  plants  and 
animals.  In  Paramecium  the  nucleus  and  cytoplasm  divide 
into  two  parts,  so  that  by  cell  division,  here  called  BINARY 
FISSION,  the  identity  of  the  parent  organism  is  merged  into 


ORIGIN    OF   THE    INDIVIDUAL  213 

the  two  new  cells.  Simple  as  this  seems,  the  fission  of  Para- 
mecium,  for  instance,  involves  considerably  more  than  the 
halving  of  the  original  cell,  because,  as  a  matter  of  fact,  each 
half  must  reorganize  into  a  complete  new  individual  with  all 
parts  characteristic  of  the  parent.  (Fig.  11.) 

Among  some  unicellular  organisms  (e.g.,  Sphaerella)  the 
parent  cell,  instead  of  merely  forming  two  cells  by  binary 
fission,  becomes  resolved  into  from  four  to  several  hundred 
cells  by  a  series  of  practically  simultaneous  divisions  known 
as  MULTIPLE  FISSION,  or  SFORULATioN.  This  is  usually  pre- 
ceded by  a  considerable  growth  of  the  parent  cell  and  its 


A  B  C  D 

FIG.  114.  —  Yeast  cells,  very  highly  magnified.  A,  cell  showing  granular 
cytoplasm  and  a  large  vacuole;  B,  showing  nucleus;  C,  cell  budding;  D,  mother 
cell  and  bud  after  division  is  completed. 

enclosure  in  a  protective  covering,  or  CYST,  which  ruptures 
to  liberate  the  spores.  Other  unicellular  forms,  such  as  the 
Yeasts  —  colorless  plants  chiefly  responsible  for  alcoholic 
fermentation — exhibit  a  modified  form  of  fission,  in  which  the 
parent  cell  forms  one  or  several  outgrowths,  'or  BUDS,  which, 
gradually  assuming  the  characteristic  adult  structures,  are 
usually  detached  as  complete  similar  individuals.  (Fig.  114.) 
In  a  considerable  number  of  instances,  however,  the  cells 
arising  by  multiple  fission  or  budding  remain  closely  asso- 
ciated or  organically  connected  so  that  they  form  a  COLONY. 
In  some  colonial  organisms  the  component  cells  are  all  alike 
and  each  retains  its  individuality,  while  in  others  certain  cells 
are  restricted  more  or  less  in  their  functions,  so  that  a  phys- 
iological division  of  labor  is  established  which  involves  the 


214 


FOUNDATIONS   OF   BIOLOGY 


shifting  of  individuality  from  the  cells  to  the  colony  as  a 
whole.  This  specialization  is  exhibited  chiefly  with  regard 
to  reproduction  and  reaches  its  highest  expression  among 
colonial  PROTISTA  (Protozoa  and  Protophyta)  in  VOLVOX, 
where  among  ten  thousand  or  so  cells,  perhaps  a  score  are 
specialized  for  reproduction  and  the  rest  are  vegetative. 
Usually  each  of  the  reproductive  cells  (germ  cells)  divides 

A.   Paramecium. 


CELL  DIVISION 

TEMPORARY 

CELL  DIVISION 

CELL  DIVISION 

(Binary  Fission) 

CONJUGATION 

(Period  of  Uo- 

(Binary  Fission) 

An  indefinite  number 

(Fertilization) 

construction) 

An  indefinite 

of  generations 

Each  cell  fer- 

Each fertilized 

number  of  gen- 

tilizes the  other. 

cell  gives  rise  to 

erations, 

four  typical 

etc. 

animals. 

B.   Volvox. 


CELL  DIVISION  CELL  DIVISION  CELL  DIVISION  PERMANENT     CELL  DIVISION 
(Colony  for-       (Asexual   Re-      (Gamete  for-      CJN.IUGA-      (Colony  forma- 

mation)  production)  mation)  TION  tion) 

Zygote  (z)  de-    Germ  cells  (g.c.)  Certain  germ  (Fertilization)  Zygote  develops 
velops  into  a      give  rise  to  new  cells  produce      One  sperm       into  a  colony, 
colony.  colonies.       eggs  (e) ;  others  fuses  with  one  etc. 

sperm,  (sp.)     egg,  forming 
a  zygote  (z). 


ORIGIN    OF   THE    INDIVIDUAL 


215 


C.   Hydra. 


ec.en.g.c. 


CELL  DIVI- 

BUDDING 

CELL  DIVISION 

PERMANENT 

CELL  DIVI- 

SION 

(Asexual  Re- 

(Gamete for- 

CONJUGA- 

SION 

(Embryological 

production) 

mation) 

TION         (Embryological 

development) 

Part  of  animal 

Certain  germ 

(Fertiliza- 

develop- 

Zygote (z) 

separates  from 

cells  produce 

tion) 

ment) 

produces  ani- 

parent and 

one  egg  (e)  and 

One  sperm 

Zygote  (z) 

mal  contain- 

leads separate 

polar  bodies 

unites  with 

produces  ani- 

ing germ  cells 

existence. 

(pb.)  ;   others 

one  egg,  form- 

mal, etc. 

(g.c.)  and  two 

produce  many 

ing  a  zygote 

layers  of  spe- 

sperm (sp.). 

GO 

cialized  somat- 

ic cells,  the 

ectoderm  (ec.) 

and  endoderm 

(en.). 

D.   Earthworm 

2 

g.c. 

pb.e 

z 

1 

"2§iP$§ic*N. 

.xfiSSI^fcSv. 

i     & 

i 

PERMANENT  CELL  DIVISION 

CONJUGATION  (Embryological 

(Fertilization)  development) 

One  sperm  unites  Zygote  (z)  pro- 

with  one  egg,  duces  animal, 

forming  a  zygote  etc. 
(z). 


CELL  DIVISION  CELL  DIVISION 

(Embryological  (Gamete  forma- 

development)  tion) 

Zygote  (z)  pro-  Certain  germ 

duces  animal  cells  produce  one 

containing  germ  egg  (e)  and 

cells  (g.c.)  and  polar  bodies 

three  layers  of  (pb.);   others 

specialized  produce  many 

sojnatic  cells,  sperm  (sp.). 
the  ectoderm 
(ec.),  mesoderm 
(ms.),  and  endo- 
derm (en.). 

FIG.  115.  —  Diagrams  to  illustrate  the  general  reproductive  cell  cycle  in  (A)  a  uni- 
cellular organism  (Paramecium) ;  (E)  a  colony  of  cells  (Volvox);  (C)  a  simple  Metazoon 
(Hydra);  and  (D)  a  more  complex  Metazoon  (Earthworm).  (From  Hegner.) 


216  FOUNDATIONS   OF   BIOLOGY 

to  form  a  group  which  is  set  free  as  a  miniature  colony;  but 
in  certain  cases  some  of  the  reproductive  cells  become  trans- 
formed into  male  and  others  into  female  gametes.  After 
fertilization  of  the  eggs,  usually  by  sperm  from  another 
colony,  the  zygotes  develop  into  new  colonies  which  even- 
tually are  liberated  from  the  parent  colony.  (Fig.  18.) 

As  has  been  previously  suggested,  the  physiological  divi- 
sion of  labor  in  the  colonial  Protista,  involving,  as  it  does,  a 
segregation  of  reproductive  from  vegetative  structures, 
affords  a  logical  transition  from  the  unicellular  condition  to 
that  characteristic  of  the  multicellular  forms.  These,  to  all 
intents  and  purposes,  may  be  considered  highly  complex 
colonies  of  cells  in  which  specialization,  no  longer  confined 
merely  to  demarking  germinal  and  vegetative  regions,  has 
transformed  the  latter  into  a  complex  of  tissues  and  organs, 
the  body  (SOMA)  of  the  individual,  while  the  germinal  tissue 
(GERM)  is  confined  to  the  essential  reproductive  organs. 

It  is  customary,  therefore,  to  draw  a  more  or  less  sharp 
distinction  between  the  soma  and  germ  —  to  consider  the 
soma  the  individual  which  harbors,  as  it  were,  the  germ  des- 
tined to  continue  the  race.  This  theory  of  GERMINAL  CON- 
TINUITY, which  is  chiefly  associated  with  the  name  of  Weis- 
mann,  recognizes  that  the  germ  contains  living  material 
which  has  come  down  in  unbroken  continuity  ever  since  the 
origin  of  life  and  which  is  destined  to  persist  in  some  form  as 
long  as  life  itself.  On  the  other  hand,  the  soma  may  be  said 
to  arise  anew  in  each  generation  as  a  derivative  or  offshoot  of 
the  germ;  and,  after  playing  its  part  for  a  while  as  the  vehicle 
of  the  germ,  to  pass  the  germ  on  at  reproduction,  and  then 
die.  The  germinal  continuity  concept  has  altered  the  attitude 
of  biologists  toward  certain  fundamental  questions  in  heredity 
and  evolution,  as  will  be  apparent  when  these  subjects  are 
considered.  (Figs.  115,  135.) 


ORIGIN   OP   THE   INDIVIDUAL 


217 


FIG.  116.  —  Hydra  reproducing  asexu- 
ally  by  dividing  lengthwise.  (After 
Koelitz.) 


Though  Volvox  and  other  colonial  forms  afford  a  glimpse 
of  the  conditions  which  probably  prevailed  when  the  evolu- 
tionary bridge  from  unicellu- 
lar to  multicellular  organ- 
isms was  crossed,  the  varied 
methods  of  reproduction  of 
the  latter  by  no  means  in- 
dicate the  early  establish- 
ment of  a  hard  and  fast 
boundary  between  soma  and 
germ.  Many  of  the  In- 
vertebrates, such  as  Hydra 
and  various  types  of  worms, 
reproduce  not  only  by  germ  cells,  but  also  by  strictly  asexual 
processes  which  are  known  as  FISSION  and 
BUDDING.  These  processes  are  comparable 
merely  in  a  superficial  way  with  the  similarly 
named  methods  in  the  Protista.  In  some 
forms  the  whole  complex  body  divides  into  two 
or  more  parts,  each  of  which  reforms  — 
REGENERATES  —  what  was  lost  and  so  becomes 
a  complete  though  a  smaller  individual.  In 
other  forms,  as  well  as  in  Hydra  itself,  buds 
arise  as  outgrowths  from  the  body  and  develop 
into  replicas  of  the  parent  either  before  or  after 
becoming  detached.  (Figs.  116,  117.) 

In  many  of  the  nearest  allies  of  Hydra  the 
buds  remain  permanently  attached  so  that 
eventually  a  large  colony  of  organically  con- 
nected hydra-like  individuals  (HYDRANTHS)  is 
formed.  (Fig.  64.)  This  condition  leads  to  a 
physiological  division  of  labor  between  the  various  hydranths 
which  may  become  more  or  less  modified  in  structure  so  that, 


FIG.  117.— An 
unsegmented 
worm  (Flat- 
worm)  in  pro- 
cess of  fission. 
(After  Child.) 


218 


FOUNDATIONS    OF   BIOLOGY 


for  instance,  feeding,  protective,  and  reproductive  individuals 
are  established,  and  thereby  the  HYDROID  COLONY  exhibits 
what  is  termed  POLYMORPHISM.  Our  present  interest  is  confined 


FIG.  118,  —  Life  history  of  Obelia.  A,  portion  of  a  colony:  1,  ectoderm;  2,  endo- 
dsrm;  3,  mouth;  4,  enteric  cavity;  5,  stalk  of  colony;  6,  7,  and  10,  exoskeleton;  8~, 
reproductive  hydranth  (blastostyle) ;  9,  medusa  bud.  B,  free  swimming  medusa: 
1,  mouth;  2,  tentacles;  S,  reproductive  organs;  4<  radial  canals;  5,  sense  organ. 
C,  ciliated  larva  of  closely  related  species.  (From  Hegner,  after  Parker  and  Haswell, 
Shipley  and  MacBride,  and  Allman.) 

to  the  reproductive  hydranths,  which  in  many  of  the  Hydroids 
are  so  modified  that  they  are  dependent  upon  the  colony 
as  a  whole  for  all  the  necessities  of  life  and  are  merely  bodies 
which  form  by  budding  other  individuals  known  as  MEDUSAE. 


ORIGIN    OF   THE    INDIVIDUAL 


219 


The  medusae,  which  become  detached  and  swim  away, 
usually  bear  no  superficial  resemblance  to  any  of  the  other 
individuals  of  the  colony  on  which  they  arose,  but  a  study 
of  their  structure  shows  that  they  are  built  on  the  same 
fundamental  plan  and  are,  to  all  intents  and  purposes,  free- 
swimming  sexual  hydranths,  some  of  which  produce  sperm 
and  others  eggs.  The  medusae  liberate  their  sexual  products 
in  the  water  where  fertilization  occurs,  and  the  zygote  gives 


FIG.  119.  —  Diagrams  to  show  the  fundamentally  similar  structure  of  Hydra  or 
of  a  hydranth  of  Obelia  (A)  and  of  a  medusa  (B).  circ,  circular  canal;  ect,  ectoderm; 
end,  endoderm;  ent.  cav,  enteric  cavity;  hyp,  mnb,  region  of  mouth  (mth);  msgl,  meso- 
gloea;  nv,  nvl,  nerve  rings;  rad,  radial  canal;  v,  velum.  (From  Parker.) 

rise  to  a  free  swimming  embryo  (LARVA)  .  This  soon  becomes 
attached  to  some  submerged  object  and  develops  into  a  Hy- 
droid  colony.  (Figs.  118,  119.) 

Thus  the  common  Hydroids,  such  as  OBELIA,  exhibit  two 
distinct  phases,  or  generations,  in  their  life  history  —  the 
fixed,  polymorphic  colony  of  hydranths,  or  polypes,  which  is 
produced  sexually  but  is  itself  asexual;  and  the  free-swim- 
ming medusae  which  are  produced  asexually  but  are  them- 
selves sexual.  The  asexual  and  sexual  generations  alternate 
with  each  other  in  regular  sequence,  so  that  an  alternation  of 


220  FOUNDATIONS   OF   BIOLOGY 

generations  known  as  metagenesis  occurs,  which,  thougn  it 
differs  from,  recalls  the  conditions  which  obtain  in  plants. 

Alternation  of  asexual  and  sexual  methods  of  reproduction, 
attended  by  more  or  less  difference  in  structure  of  the  indi- 
viduals of  the  generations,  is  fairly  widespread  among  the 
Invertebrate  groups,  particularly  in  forms  which  have 
adopted  a  parasitic  mode  of  life.  Frequently  the  life  his- 
tories are  exceedingly  complicated:  several  asexual,  sexual, 
and  parthenogenetic  generations  succeeding  one  another  in 
response  to  the  exigencies  imposed  by  adaptation  to  a  life 
within  another  animal  or  series  of  animals. 

It  is  clear  from  such  life  histories  that  the  conception  of 
special  germ  cells  early  set  aside,  as  it  were,  from  the  somatic 
cells  must  not  be  taken  too  literally.  The  same  point  is 
emphasized  by  the  power  exhibited  by  plants  and  animals 
in  restoring  parts  lost  by  mutilations  of  one  kind  or  another. 
Among  many  plants,  pieces  of  the  root,  stem,  or,  in  special 
cases,  of  the  leaf  may  give  rise  to  individuals  complete  in  every 
respect.  Until  the  middle  of  the  eighteenth  century  this  was 
considered  a  property  peculiar  to  plants,  and  accordingly 
soon  after  Hydra  was  discovered  experiments  were  made  to 
determine  whether  the  organism  was  a  plant  or  an  animal. 
Specimens  were  cut  into  several  pieces  and  it  was  found  that 
each  piece  developed  into  a  complete  Hydra.  This  result, 
from  the  ideas  of  the  time,  should  have  led  to  the  conclusion 
that  Hydra  is  a  plant,  but  additional  characteristics  were  ob- 
served which  outweighed  all  other  considerations.  Accord- 
ingly Hydra  was  recognized  as  an  animal  with  the  power  of 
replacing  lost  parts.  (Fig.  120.) 

Since  the  classic  work  on  Hydra  the  power  of  regeneration 
has  come  to  be  recognized  as  a  fundamental  property  of  all 
animals.  It  is  exhibited  to  the  greatest  degree  among  the 
lower  animals  while  in  the  higher  Vertebrates  it  is  confined 


ORIGIN    OF  THE   INDIVIDUAL  221 

chiefly  to  the  replacement  of  cells  which  especially  suffer  from 
wear  and  tear,  such  as  those  forming  the  outer  layer  of  the 
skin.  It  will  be  recognized  that  regeneration  is  but  one 
phase  of  a  fundamental  property  of  protoplasm,  namely 
growth,  whether  it  consists  in  restoring  a  part  of  a  Parame- 
cium,  transforming  a  bit  of  a  Flatworm  into  a  complete 
animal,  or  replacing  half  of  an  Earthworm,  the  head  of  a 


•  •ft  I 


FIG.  120.  —  Regeneration  and  grafting  in  Hydra.  A,  an  individual  with 
seven  '  heads'  as  a  result  of  lengthwise  cuts.  B,  stages  in  the  regeneration  of  a 
complete  individual  from  a  small  piece.  C,  Portions  of  two  individuals  grafted 
together.  (From  Hegner;  A,  after  Trembley;  B,  after  Morgan;  C,  after  King.) 

Snail,  the  claw  of  a  Crayfish,  or  the  leg  of  a  Salamander.  But 
the  experimental  study  of  regeneration  phenomena  has 
opened  up  a  new  vista  of  the  regulatory  powers  of  living 
things  from  Protist  to  Vertebrate  and  from  egg  to  adult,  and 
has  afforded  a  means  of  approach  to  some  fundamental  bio- 
logical problems.  And  withal  it  has  a  practical  value.  The 
surgeon  now  knows  more  of  the  regeneration  of  tissues  in 
general  and  nerves  in  particular  in  wound  healing,  and  the 
oysterman  knows  —  or  should  know  —  that  his  attempt  to 
destroy  Starfish  by  tearing  them  up  and  throwing  the  pieces 


222 


FOUNDATIONS   OF  BIOLOGY 


overboard,  serves  merely  to  increase  many  fold  this  enemy 
of  the  oyster.     (Figs.  121,  122.) 

The  power  of  fragments  of  distinctively  somatic  tissue,  as 
in  the  Earthworm  and  many  plants,  to  form  a  complete 
organism  including  the  reproductive  organs  and  germ  cells, 
indicates  that  we  must  postulate  at  least  a  potential  supply 


i\ 


\j 


FIG.  121.  —  Regeneration  and  grafting  in  the  Earthworm.  A,  regeneration  of  re- 
moved anterior  segments  by  the  posterior  piece.  B,  regeneration  of  posterior  seg- 
ments by  the  posterior  part,  so  that  the  worm  has  a  'tail'  at  either  end.  C,  regenera- 
tion of  removed  posterior  end  by  the  anterior  piece.  D,  three  pieces  grafted  together  to 
make  a  long  worm;  E,  two  pieces  grafted  to  form  a  worm  with  two  'tails';  F,  short 
anterior  and  posterior  pieces  grafted  together.  Regenerated  portions  are  dotted. 
(From  Hegner,  after  Morgan.) 

of  the  germ  residing  in  the  somatic  tissue,  which  can  make 
good  the  definitive  germ  cells  when  they  are  lost.  At  first 
glance  this  may  seem  to  be  a  far  cry  to  save  an  idea,  but  it  is 
a  fact  that  there  is  a  continuity  of  the  nuclear  complex  (GERM 
PLASM)  whether  the  germ  cells  are  set  aside  early  in  individual 
development,  or  later  by  the  transformation  of  what  seem  to 
be  typical  somatic  cells.  That  this  is  really  the  crux  of  the 
question  will  be  appreciated  after  the  details  of  cell  division 
have  been  described. 


ORIGIN   OP  THE   INDIVIDUAL 


223 


C.   ORIGIN  OF  THE  GERM  CELLS 

Among  the  Vertebrates,  as  we  know,  the  germ  cells  reside 
during  adult  life  in  definite  organs,  the  ovaries  and  testes,  and 
upon  these  cells  the  power  of  reproduction  of  the  individual 
is  solely  dependent.  It  seems  clear,  however,  that  the 


I 


B' 


I 
t 
$& 


C' 

FIG.  122.  —  Regeneration  of  a  Flatworm  (Planaria  maculata).  A,  normal 
worm;  cut  across  at  line  indicated.  B,  B',  and  C,  C',  regeneration  of  an- 
terior and  posterior  parts  of  A  to  form  complete  worms.  D,  piece  cut  from  a 
worm;  Dl,  Z>2,  D3,  Z>4,  successive  stages  in  the  regeneration  of  D.  E,  'head'  from 
which  rest  of  animal  has  been  cut  off.  E1,  E2,  Es,  successive  stages  in  the  re- 
generation by  E  of  a  complete  body.  F,  similar  experiment  to  E,  but  a  new 
'head'  in  reversed  position  is  regenerated  instead  of  a  body,  Fl.  (From  Hegner, 
after  Morgan.) 

primordial  germ  cells  do  not  arise  as  such  by  division  in  the 
tissues  which  during  development  form  the  ovaries  and  testes. 
Just  when  the  germ  cells  are  set  aside  in  Vertebrates  is  un- 
certain, but  it  would  seem  to  occur  very  early  in  embryonic 
life,  perhaps  during  the  cleavage  of  the  egg.  Then  by 
shif tings  of  the  tissues  during  growth,  and  possibly  also  by 
amoeboid  movements  of  the  germ  cells  themselves,  they 
finally  reach  definite  positions  in  the  epithelium  lining  the 


224  FOUNDATIONS   OF   BlOLOGi 

dorsal  wall  of  the  coelom,  which  becomes  an  integral  part  of 
the  gonads  as  development  proceeds. 

With  regard  to  the  fate  of  the  PRIMORDIAL  GERM  CELLS, 
once  they  have  reached  testis  or  ovary,  we  are  on  surer  ground 
and  can  trace  with  considerable  exactness  their  divisions  and 
transformations  which  give  rise  to  the  gametes,  sperm  and 
eggs.  In  the  first  place  the  primordial  germ  cells  proceed  to 
multiply  in  the  testis  and  ovary  so  that  they  produce  a  large 
number  of  relatively  small  germ  cells  known  as  SPERMATO- 
GONIA  and  OOGONIA  respectively. 

1.   Mitosis 

Before  taking  up  the  origin  of  the  gametes  from  the  sper- 
matogonia  and  oogonia,  it  will  be  necessary  to  describe  in 
some  detail  the  complicated  internal  process  involved  in  all 
typical  cell  divisions,  known  as  MITOSIS,  which  was  dismissed 
when  considering  the  origin  of  cells  until  the  reader  would  be 
in  a  position  to  appreciate  to  the  full  its  significance. 

Reduced  to  its  simplest  terms,  a  typical  resting  cell,  that 
is  one  which  is  not  dividing,  consists  of  a  mass  of  cytoplasm 
surrounding  a  nucleus;  the  latter  with  its  chromatin  dis- 
tributed so  that  it  presents  a  net-like  appearance.  In  addi- 
tion to  the  nucleus,  it  will  be  recalled  that  there  is  present 
another  important  cell  organ,  the  CENTROSOME,  which  ap- 
pears like  a  tiny  granule  situated  in  the  cytoplasm  near  the 
nucleus  of  the  resting  cell.  For  all  practical  purposes  we  may 
consider  the  cytoplasm  as  the  arena  in  which  mitosis  takes 
place,  the  centrosome  as  the  dynamic  agent,  and  the  nucleus, 
or  more  specifically  its  chromatin,  as  the  essential  element 
which  the  complicated  process  is  particularly  designed  to 
distribute  with  nicety  to  the  daughter  cells  which  are  in  pro- 
cess of  formation.  With  this  in  mind  we  may  proceed  to  an 
outline  of  the  chief  stages  of  mitosis,  first  cautioning  the  reader 


ORIGIN    OF   THE    INDIVIDUAL 


225 


to  remember  that  variations  in  the  details  are  as  numerous  as 
the  different  types  of  cells,  and  that  any  general  account  can 
do  no  more  than  present  the  fundamental  plan  of  operations. 


FIG.  123.  —  Diagrams  of  typical  stages  in  mitosis.  A,  resting  cell  with  chromatin 
presenting  a  net-like  arrangement  within  the  nuclear  membrane;  c,  centrosome  divided; 
B,  Prophasc  (early):  centrosomes,  asters  (a),  and  spindle;  most  of  the  chromatin 
material  seems  to  assume  the  form  of  a  long  thread  (spireme);  C,  prophase  (later) 
involving  the  disappearance  of  the  nuclear  membrane,  and  the  separation  of  the  chro- 
matin of  the  spireme  stage  into  discrete  bodies  (chromosomes);  D,  prophase  (final) 
with  chromosomes  arranged  in  the  equatorial  plate  (ep);  E,  metaphase;  each  chromo- 
some splitting  lengthwise;  F,  anaphase:  the  daughter  sets  of  chromosomes  moving 
toward  the  asters;  if,  'inter-zonal  fibers';  G,  H,  early  and  later  telophase  involving  the 
gradual  loss  of  visibility  of  chromosomes  as  they  spin  out  into  the  resting  net-like 
arrangement  of  the  chromatin;  division  of  the  cytoplasm;  n,  nucleolus.  (After  Wilson.) 

Broadly  speaking,  mitosis  can  be  divided  into  four  chief 
stages:  PROPHASE,  METAPHASE,  ANAPHASE,  and  TELOPHASE, 
during  each  of  which  characteristic  changes  take  place  in 
the  nucleus,  cytoplasm,  and  centrosome.  (Figs.  8,  123.) 


226  FOUNDATIONS   OF   BIOLOGY 

At  the  beginning  of  the  prophase,  or  earlier,  the  centrosome 
divides  to  form  two,  each  of  which  becomes  surrounded  by 
what  appears  to  be  a  halo  (ASTER)  of  radiating  fibers  which 
are  possibly  cytoplasmic  currents  —  the  visible  expression  of 
physico-chemical  forces.  The  centrosomes  and  asters  now 
proceed  to  move  apart,  take  up  positions  at  opposite  sides 
of  the  nucleus,  and  the  astral  fibers  between  lengthen  until 
they  form  a  CENTRAL  SPINDLE.  While  these  changes  are 
going  on,  the  nucleus  is  not  inactive.  The  nuclear  membrane 
gradually  disappears  and  the  chromatin  granules,  originally 
in  a  net-like  arrangement,  seem  to  become  rearranged  in  a 
more  or  less  continuous  thread  of  chromatin  called  the 
SPIREME.  This,  however,  actually  represents  a  number  of 
definite  chromatin  entities,  termed  CHROMOSOMES,  which 
gradually  by  chromatin  concentration  become  distinctly  in- 
dividual. The  number  of  chromosomes  varies  greatly  in 
different  species,  but  is  typically  an  even  number  and  the 
same  for  all  the  cells  of  a  given  species. 

When  the  chromosomes  have  assumed  definitive  form,  the 
preliminary  events  which  constitute  the  prophase  of  mitosis 
are  brought  to  a  close  by  the  chromosomes  being  drawn  to 
the  center  of  the  spindle.  Here  they  are  arranged  in  a  plane 
at  right  angles  to  the  long  axis  of  the  central  spindle,  midway 
between  the  two  centrosomes,  and  form  the  EQUATORIAL 
PLATE. 

And  now  the  stage  is  set  for  what  is  apparently  the  climax 
of  mitosis,  designated  the  metaphase.  Each  of  the  chromo- 
somes separates  into  two  parts  along  the  line  of  a  longitudinal 
split, in  such  a  manner  that  each  of  the  thousands  of  chromatin 
granules  which  make  up  a  chromosome  is  equally  divided. 
Two  sets  of  similar  daughter  chromosomes  are  thus  formed. 

With  chromosome  division  consummated,  the  metaphase 
merges  into  the  anaphase  which  is  devoted  to  a  shifting  of  a 


ORIGIN    OF   THE    INDIVIDUAL  227 

daughter  set  of  chromosomes  along  the  fibers  to  either  end 
of  the  spindle.  In  this  way  each  centrosome  becomes  asso- 
ciated with  one  set  of  daughter  chromosomes. 

The  last  stage,  or  telophase,  is  one  of  nuclear  reconstruc- 
tion and  division  of  the  cytoplasm.  The  chromosomes  be- 
come indistinct  as  they  spin  out  to  form  the  net-like  arrange- 
ment of  the  chromatin  in  the  nucleus  of  each  daughter  cell; 
a  nuclear  membrane  arises;  and  the  nucleus  again  assumes 
the  form  of  a  definite  spherical  body  characteristic  of  the 
resting  cell.  It  must  be  emphasized,  however,  that  although 
the  chromosomes  usually  disappear  from  view  as  definitive 
entities  in  the  resting  nucleus,  nevertheless  the  individuality 
of  each  persists  and  the  same  chromosomes  emerge  from  the 
nuclear  complex  at  the  next  division  period. 

Simultaneously  with  these  nuclear  changes,  and  before  the 
spindle  and  asters  —  the  machinery  of  mitosis  —  disappear, 
the  division  of  the  cytoplasm  is  initiated  as  indicated  by  an 
indentation  of  the  cell  wall  at  the  equator  of  the  cell.  This 
gradually  extends  through  the  cytoplasm  in  the  same  plane 
which  the  equatorial  plate  formerly  occupied,  until  the  cyto- 
plasm is  cut  into  two  separate  masses,  each  containing  one 
of  the  daughter  nuclei  and  centrosomes.  And  one  cell  has 
merged  its  individuality  into  two  daughter  cells  by  mitotic 
division. 

A  little  thought  will  convince  the  reader  that  whereas  the 
mitotic  process  apparently  results  in  merely  a  mass  division 
of  the  cytoplasm,  the  chromatin  material  is  rearranged  and 
distributed  in  a  manner  which  makes  it  possible  for  each  cell 
to  receive  a  very  definite  share.  Indeed  this  seems  to  be  the 
primary  object  of  mitosis.  For  in  many  cases  there  is  a  very 
great  difference  in  the  size  of  the  resulting  cells,  but  the  num- 
ber of  chromosomes  in  each  is  the  same.  This,  and  other 
evidence  which  will  presently  appear,  has  clearly  established 


228  FOUNDATIONS    OF    BIOLOGY 

the  chromosomes  as  the  chief  factors  in  the  transmission  of 
characters  from  cell  to  cell,  and  therefore  in  inheritance. 

2.   Gametes 

Returning  now  to  the  origin  of  gametes.  The  spermato- 
gonia  and  oogonia  in  the  reproductive  organs  are,  together 
with  all  the  cells  forming  the  body  proper,  direct  descendants 
by  mitotic  cell  division  from  the  fertilized  egg  which  gave  rise 
to  the  individual  organism.  This  is  equally  true  of  the  chro- 
mosomes themselves  and  accordingly  every  cell  of  the  animal 
has  the  same  number  of  chromosomes  as  the  fertilized  egg. 

Fertilization,  as  we  now  know,  always  consists  of  the 
fusion  of  two  gametes,  whether  it  is  in  plants  or  animals;  a 
fusion  of  nucleus  with  nucleus  and  cytoplasm  with  cytoplasm 
to  form  a  zygote,  which  therefore  is  one  cell  reconstructed  from 
two.  Such  being  the  case,  one  of  two  things  must  happen  at 
fertilization.  Either  the  fertilized  egg  must  have  double  the 
chromosome  number,  that  is  a  set  contributed  by  both  egg 
arid  sperm;  or  some  method  must  exist  by  which  the  chromo- 
somes of  the  gametes  are  reduced  in  number  to  one  half  that 
characteristic  of  the  somatic  cells. 

As  a  matter  of  fact  a  reduction  in  the  number  of  chromo- 
somes always  takes  place  sometime  during  the  life  history. 
In  plants  such  as  the  Mosses,  Ferns,  and  Flowering  Plants,  it 
occurs  at  the  formation  of  the  spores.  Thus  it  follows  that 
the  gametophyte  contains  half  as  many  chromosomes  as  the 
sporophyte,  and  the  sporophyte  number  is  restored  by  the 
union  of  the  gametes.  It  must  be  borne  in  mind,  however, 
that  the  familiar  plants  are  sporophytes  which,  for  all  prac- 
tical purposes,  directly  produce  sporophytes  because  the 
gametophyte  is  reduced  almost  to  the  vanishing  point.  The 
chromosome  number  of  the  parent  sporophyte  and  the 
sporophyte  in  the  seed  is  the  same.  But  we  cannot  digress 


ORIGIN    OF   THE    INDIVIDUAL 


229 


ADULT  ANIMAL 

Primordial 
Germ 
Cells 

Somatic 
Cells 

Diploid 

ADULT  GAMETOPHYTE 

Somatic 
Cells 

Primordial 
Germ 
Cells 

Haploid 

ADULT  SPOROPHYTE 

Primordial 
Spore 
Cells 

Somatic 
Cells 

Diploid 

FIG.  124.  —  Schematic  representation  of  the  life  history  of  an  animal  (A)  and  of  a 
plant,  e.g.,  Fern,  (B)  from  the  standpoint  of  the  diploid  and  haploid  condition  of  the 
chromosomes. 


230  FOUNDATIONS   OF   BIOLOGY 

to  elaborate  the  details  of  the  chromosome  cycle  associated 
with  alternation  of  generations  in  plants —  attention  must 
be  concentrated  on  the  conditions  as  they  exist  in  animals, 
in  which  the  somatic  number  of  chromosomes  is  reduced  one 
half  at  the  formation  of  the  gametes.  From  the  standpoint 
of  chromosome  number,  the  sporophyte  is  comparable  to 
the  animal  soma  and  the  gametophyte  is  represented  by 
merely  a  couple  of  cell  generations  during  the  formation  of 
the  gametes  in  animals.  (Fig.  124.) 

The  MATURATION  or  'ripening'  of  the  germ  cells  of  animals 
involves  two  cell  divisions  by  which  each  spermatogonium 
gives  rise  to  four  sperm,  and  each  oogonium  to  one  functional 
egg  and  three  tiny,  abortive  eggs  known  as  POLAR  BODIES; 
each  and  all  with  one  half  the  number  of  chromosomes  of  the 
somatic  cells  and  of  the  germ  cells  up  to  this  point  in  their 
developmsnt.  Consequently  these  two  divisions,  termed 
MATURATION  DIVISIONS,  must  be  examined  in  some  detail  if 
we  are  to  appreciate  the  nicety  of  the  process  by  which  the 
chromosome  number  is  reduced  one  half  without  impairing 
the  chromatin  heritage  from  cell  to  cell.  We  shall  describe 
first  the  origin  of  the  sperm  which,  though  it  is  fundamentally 
the  same  as  that  of  the  egg,  is  somewhat  simpler  to  under- 
stand. 

3.   Spermatogenesis 

A  given  SPERMATOGONIUM,  with,  let  us  say,  eight  chromo- 
somes characteristic  of  the  species,  proceeds  to  increase  in 
size  preparatory  to  the  first  maturation  mitosis,  and  is  desig- 
nated a  PRIMARY  SPERMATOCYTE.  At  the  close  of  the  growth 
period,  when  this  cell  is  preparing  to  divide,  the  chromosomes 
are  arranged  in  pairs  by  a  process  termed  SYNAPSIS.  The 
number  of  such  pairs  will  obviously  be  half  that  of  the  chro- 
mosome number.  The  synaptic  pairs  are  then  distributed  in 
the  equator  of  the  spindle  exactly  as  the  single  chromosomes 


ORIGIN    OF   THE    INDIVIDUAL 


231 


FIG.  125.  —  Diagram  of  the  general  plan  of  spermatogenesis  and  oogenesis  in  animals. 
The  somatic,  or  diploid,  number  of  chromosomes  (duplex  group)  is  assumed  to  be  eight. 
Male,  to  the  left;  female,  to  the  right.  A,  primordial  gerrn  cells;  B,  spermatogonia  and 
oogonia,  many  of  which  arise  during  the  period  of  multiplication;  C,  primary  spermato- 
cyte  and  oocyte,  after  the  growth  period,  with  chromosomes  in  synapsis;  D,  secondary 
spermatocytes  and  oocytes,  with  haploid  number  (simplex  group)  of  chromosomes, 
which  have  arisen  by  the  first  maturation  (reduction)  division;  E,  spermatids  (which 
become  transformed  into  sperm)  and  egg  and  three  polar  bodies  which  have  arisen  by 
the  second  maturation  (equation)  division;  F ,  union  of  sperm  and  egg  (fertilization) 
to  form  zygote  with  diploid  number  (duplex  group)  of  chromosomes;  C,  chromosome 
complex  of  cells  after  first  division  of  the  zygote,  and  of  all  subsequent  somatic  cells, 
and  germ  cells  until  maturation. 


232  FOUNDATIONS   OF   BIOLOGY 

are  in  ordinary  mitosis.  But,  and  this  is  the  crucial  point, 
in  the  early  anaphase  the  members  of  each  pair  are  separated, 
one  sy  nap  tic  mate  going  to  each  pole  of  the  spindle.  Thus 
each  of  the  daughter  cells  —  SECONDARY  SPERMATOCYTES  — 
receives  half  the  total  number  of  chromosomes  that  were 
present  in  the  primary  spermatocyte  or  the  somatic  cells. 
The  essential  difference  between  this  type  of  mitosis  (REDUC- 
TION DIVISION)  and  that  involved  in  other  nuclear  divisions 
(EQUATION  DIVISIONS)  lies  in  the  separation  of  entire  chromo- 
somes (synaptic  mates)  instead  of  the  splitting  of  each  chro- 
mosome. Both  the  secondary  spermatocytes  now  divide  by 
typical  mitosis,  thus  distributing  to  each  of  the  resulting  cells 
(SPERMATIDS)  half  the  somatic  number  of  chromosomes.  The 
spermatids  are  presently  transformed  into  sperm  and  thus 
each  spermatogonium  with  eight  chromosomes  gives  rise  to 
four  sperm  with  four  chromosomes  apiece.  (Fig.  125.) 

4.   0  agenesis 

The  maturation  of  the  egg,  as  already  intimated,  follows 
the  same  plan  as  that  of  the  sperm,  and  the  reduction  of  the 
chromosomes  is  the  same.  Such  modifications  as  occur  are 
related  to  the  fact  that  the  egg  is  usually  a  relatively  large 
passive  cell  stored  with  nutritive  materials  for  use  during  the 
developmental  process,  while  the  sperm  is  among  the  smallest 
of  cells  —  essentially  a  nucleus  surrounded  with  a  delicate 
envelope  of  cytoplasm.  Accordingly  it  is  only  necessary  to 
emphasize  that  the  growth  period  of  egg  formation,  in  which 
the  OOGONIUM  becomes  transformed  into  the  PRIMARY 
OOCYTE,  is  characterized  by  a  much  greater  increase  in  size 
than  is  the  case  in  the  corresponding  period  in  spermato- 
genesis;  and  that  both  of  the  ensuing  cell  divisions  (one  a 
reduction  and  the  other  an  equation  division)  involve  very 
unequal  divisions  of  the  cytoplasm.  Thus  one  SECONDARY 


ORIGIN    OF   THE    INDIVIDUAL  233 

OOCYTE  is  very  large,  while  the  other  is  a  tiny  cell  termed  the 
FIRST  POLAR  BODY.  Both  the  large  secondary  oocyte  and 
first  polar  body  now  divide  again;  the  former  giving  rise  to 
a  large  cell,  the  mature  EGG,  and  a  tiny  SECOND  POLAR  BODY; 
while  the  first  polar  body  divides  equally  to  form  two  polar 
bodies.  In  this  way  arise  the  four  cells,  comparable  to  the 
four  sperm  in  spermatogenesis,  each  with  half  the  somatic 
number  of  chromosomes.  But  only  one  of  these,  the  egg, 
functions  as  a  gamete.  The  three  polar  bodies,  although 
possessing  a  similar  chromosome  complex,  are  sacrificed  in 
providing  one  cell  with  its  special  cytoplasmic  equipment. 
The  polar  bodies  get  just  enough  cytoplasm  to  be  regarded 
as  cells,  and  soon  degenerate  and  disappear. 

Such  is  the  outline  of  the  essentials  of  spermatogenesis  and 
oogenesis  in  animals;  processes  which  involve  at  one  stage  a 
modification  of  ordinary  mitosis  to  give  each  gamete  half  the 
somatic  number  of  chromosomes  characteristic  of  the  species. 
It  is  clear  that  this  is  not  merely  a  mass  reduction  of  chromatin 
material,  but  is  a  separating  of  definite  chromatin  entities, 
the  chromosomes,  so  that  the  gametes  receive  the  reduced 

number. 

5.  The  Chromosome  Cycle 

Throughout  the  animal  kingdom,  wherever  sexual  repro- 
duction occurs,  phenomena  which  can  be  interpreted  as 
nuclear  reduction  have  been  observed  in  the  formation  of 
gametes.  In  some  of  the  Protozoa  this  seems  to  be  merely 
an  extrusion  of  a  certain  amount  of  chromatin,  but  since 
whenever  chromosomes  can  be  observed  and  counted  the 
process  has  been  found  to  follow  in  principle  essentially  the 
same  lines  described  above,  we  have  every  reason  to  believe 
that  it  is  never  a  haphazard  mass  reduction,  and  that  the  ripe 
gametes  emerge  with  a  definite  chromatin  heritage,  relatively 
simple  as  this  may  be  in  the  lowest  forms. 


234  FOUNDATIONS   OF   BIOLOGY 

We  have  now  surveyed  the  germ  cell  cycle  from  fertil- 
ized egg  through  the  germ  plasm  in  the  adult  to  the 
gametes  again,  but  before  proceeding  to  consider  the  details 
of  the  fusion  of  egg  and  sperm  —  the  fertilization  process  — 
it  may  clarify  matters  to  glance  back  to  the  chromosome 
condition  in  the  fertilized  egg  at  the  beginning  of  the  cycle 
which  has  just  been  considered.  Obviously  this  fertilized  egg 
(zygote)  contained  chromosomes,  half  of  which  belonged  to 
the  egg  and  therefore  may  be  termed  MATERNAL,  and  half  of 
which  were  derived  from  the  sperm  and  thus  are  PATERNAL. 
When  the  zygote  divided  by  mitosis  to  form  the  body  and  germ, 
every  cell  received  a  set  of  chromosomes,  directly  derived 
from  this  original  set  in  the  zygote.  It  logically  follows, 
and  all  observations  indicate,  that  each  and  every  cell,  both 
of  the  soma  and  of  the  germinal  tissue,  possesses  a  set  of  chro- 
mosomes, half  of  which  are  of  maternal  and  half  of  paternal 
origin  —  in  other  words  are  direct  lineal  descendants  of  the 
combined  set  formed  at  fertilization.  So  it  happens,  that 
each  body  cell  really  has  a  double  set  (DUPLEX  GROUP,  DIPLOID 
NUMBER)  of  homologous  chromosomes  —  and  the  same  is 
true  of  the  germ  cells  until  maturation.  Then  at  synapsis 
maternal  and  paternal  chromosomes  pair  and,  after  the  re- 
duction division,  the  secondary  spermatocytes  and  oocytes 
and  the  gametes  themselves  have  a  single  set  (SIMPLEX 

GROUP,  HAPLOID  NUMBER).       (FigS.  124A,  126.) 

Thus  far  we  have  emphasized  chromatin  and,  in  particular, 
chromosome  reduction  as  the  main  purpose  of  the  compli- 
cated maturation  phenomena.  The  question  now  arises:  Is 
this  chromatin  distributed  so  that  all  the  gametes  receive  the 
same  heritage? 

All  the  evidence  at  hand  indicates  not  only  that  chromo- 
somes differ  qualitatively  one  from  another  but  also  that 
the  various  parts  (CHROMOMERES)  of  each  chromosome  are 


ORIGIN    OF   THE    INDIVIDUAL 


235 


ABCD 


JL 


JOL 


AaBbCcDd 


eiBCd. 


AbcD 


FIG.  126. — Diagram  of  the  chromosome  cycle  of  an  animal.  Somatic  (diploicl) 
chromosome  number  assumed  to  be  eight.  Paternal  chromosomes  (from  sperm)  = 
A  B  C  D;  •maternal  (from  egg)  =  a  b  c  d.  I,  union  of  nuclei  of  gametes,  each  with  a  sim- 
plex group  (haploid  number)  of  chromosomes,  in  the  zygote  nt  fertilization  to  form  a 
duplex  group  (diploid  number)  of  chromosomes.  II,  III,  IV,  somatic  divisions  or  divi- 
sions of  germ  cells  before  maturation  (duplex  groups  of  chromosomes).  V,  synapsis, 
involving  pairing  of  homologous  paternal  and  maternal  chromosomes  to  give  the 
haploid  number  of  paired  chromosomes.  VI,  reduction  division  —  separation  of  pairs 
into  single  chromosomes  again.  VII,  two  gametes,  with  simplex  groups  (haploid 
number)  of  chromosomes;  there  are  14  more  possible  combinations  of  the  chromosomes, 
or  types  of  gametes,  which  are  not  shown.  (After  Wilson,  slightly  modified.) 


236 


FOUNDATIONS   OF   BIOLOGY 


qualitatively  distinct.  And  further  that  these  qualitative 
differences  are  the  physical  basis  of  inheritance  —  the  de- 
terminers (GENES)  of  characters  which  will  be  realized  in  the 
individual  or  the  race  to  which  the  cell  containing  them  con- 
tributes. Such  being  the  case,  the  chromosomal  complex  of 
the  nuclei  which  arises  after  synapsis  —  that  is,  the  nuclei 


FIG.  127.  —  A,  section  through  the  egg  of  a  primitive  Vertebrate,  the  Lamprey.  B 
sperm  of  the  same  species,  drawn  to  scale,  d.en,  dense  endoplasm;  i.m,  inner  membrane 
o.m,  outer  membrane;  p,  granular  'polar'  cytoplasm;  v.en,  vacuolated  endoplusm 
T.ex,  vacuolated  ectoplasm;  /,  first  polar  body;  II,  spindle  for  second  polar  body 
(From  Kellicott.) 

of  the  gametes  —  depends  on  how  the  various  chromosomes 
happen  to  be  distributed  during  the  two  maturation  divisions. 
As  a  matter  of  fact  all  the  chromosomal  combinations  occur 
which  are  mathematically  possible  with  the  available  num- 
ber of  chromosomes  in  a  given  species,  but  with  one  limita- 
tion: every  cell  must  receive  one  member  of  each  synaptic 
pair  of  chromosomes,  so  that  each  and  every  gamete  receives  a 
complete  simplex  group  of  chromosomes,  but  rarely  the  same 


ORIGIN   OF   THE    INDIVIDUAL  237 

groups  (maternal  and  paternal)  which  existed  before  matura- 
tion. For  example,  if  the  somatic  (diploid)  number  of  chro- 
mosomes is  eight,  sixteen  different  types  of  gametes  are 
possible.  In  Man  with  48  somatic  chromosomes,  and 
after  synapsis  24  pairs  of  paternal  and  maternal  chromosomes, 
there  are  224,  or  about  twenty  million  possible  types  of 
gametes  in  each  sex,  and  since  these  combine  at  random  at 
fertilization,  the  number  of  possible  different  types  of  zygotes 
from  one  parental  pair  mounts  far  up  in  the  trillions.  No 
wonder  the  children  of  a  family  differ  —  there  is  variation! 

In  a  way,  therefore,  fertilization  is  not  consummated,  so 
far  as  its  influence  on  the  race  is  concerned,  until  the  matura- 
tion of  the  gametes  in  the  new  generation.  We  must  defer 
until  later  the  consideration  of  the  significance  of  these  facts 
in  biparental  inheritance,  and  take  up  now  some  necessary 
details  of  the  gametes  themselves  and  of  how  they  unite  to 
form  the  zygote. 

6.   Fertilization 

The  gametes,  while  exhibiting  in  certain  cases  peculiar 
adaptations  to  special  conditions,  are  remarkably  similar  in 
general  structure  throughout  the  animal  series.  It  is  possible 
in  animals,  just  as  in  plants,  to  arrange  a  series  of  lower  forms 
which  shows  various  stages  in  sex  differentiation.  Beginning 
with  animals  in  which  both  gametes  are  structurally  similar, 
we  pass  by  slow  gradations  to  others  in  which  the  egg  is  a 
relatively  large,  passive,  food-laden  cell  and  the  sperm  a 
minute,  active,  flagellated  cell.  As  a  matter  of  fact  the  egg 
is  subject  to  somewhat  more  variation  in  size  and  general 
appearance  than  the  sperm,  for  after  fertilization  it  must 
be  adapted  to  meet  the  special  conditions  of  development 
peculiar  to  the  species.  Thus,  for  instance,  the  actual  size 
of  the  egg  in  both  plants  and  animals  is  determined  chiefly  by 


238  FOUNDATIONS   OF   BIOLOGY 

whether  the  developing  embryo  is  in  the  main  dependent  upon 
food  stored  in  the  cytoplasm  of  the  egg  itself,  or  upon  some 
outside  source,  such  as  the  sea  water  in  which  it  floats,  or  the 
tissues. of  the  parent.  The  first  case  is  well  illustrated  by  a 
Bird's  egg  in  which  the  so-called  YOLK  is  the  egg  cell  proper, 
hugely  distended  by  stored  food,  and  surrounded  by  nutritive 


FIG.  128.  —  Diagram  of  the  egg  of  the  domestic  Fowl,  before  incubation,  a,  air 
space  between  two  layers  of  shell  membrane;  alb,  ch,  dense  albumin  (chalaza);  alb', 
more  fluid  albumin  (white  of  egg) ;  bl,  point  of  cytoplasmic  concentration  from  which 
embryo  arises  (blastoderm) ;  sh,  shell;  shm,  shell  membrane.  (After  Marshall.) 

and  protective  envelopes  consisting  of  the  ' white  of  the  egg/ 
shell  membranes,  and  shell  which  are  formed  by  secretions 
from  the  walls  of  the  oviduct  during  the  passage  of  the  egg  to 
the  exterior.  On  the  other  hand  the  eggs  of  Mammals,  for 
instance  of  the  Rabbit  and  Man,  are  very  small  —  the  human 
egg  being  less  than  1/1 25th  of  an  inch  in  diameter  —  since 
their  essentially  parasitic  method  of  development  in  the  uterus 
renders  superfluous  the  storage  of  any  considerable  amount 
of  food  material  in  the  egg  cytoplasm.  (Figs.  127-129.) 


ORIGIN    OF   THE    INDIVIDUAL 


239 


With  the  specialization  of  the  egg  along  lines  which  render 
it  non-motile,  it  has  devolved  upon  the  sperm  to  assume  the 
function  of  seeking  out  the  egg  for  fertilization.  It  does  this 


FIQ.  129.  —  Human  egg  cell,  X  415,  and  sperm  cell,  X  2000.  A,  Egg  just 
removed  from  the  ovary,  surrounded  by  follicle  cells  of  the  ovary  and  a  clear 
membrane.  The  central  part  of  the  egs;  contains  metaplasmic  bodies  and  the 
large  nucleus.  Superficially  there  is  a  clear  ectoplasmic  region.  (After  Wal- 
deyer.)  B,  two  views  of  the  human  sperm,  c,  centrosome;  h,  'head'  consisting 
of  the  nucleus  surrounded  by  a  cytoplasmic  envelope;  m,  ne,  middle  piece;  t, 
tail  or  flagellum.  (After  Retzius.) 


in  most  cases  by  active  lashing  of  its  flagellum.  This  necessi- 
tates a  fluid  medium  in  which  the  sperm  can  swim,  and  such 
is  provided  by  the  environment  in  which  the  organism  lives 
or,  in  the  case  of  most  higher  animals,  where  fertilization 


240  FOUNDATIONS   OF   BIOLOGY 

takes  place  within  the  oviduct,  by  special  fluids  secreted  for 
the  purpose.  In  the  highest  plants,  however,  it  will  be 
recalled  that  the  characteristic  motility  of  the  sperm  is  lost 
in  the  excessive  specialization  attendant  upon  gametophyte 
reduction  —  the  sperm  nucleus  reaching  the  egg  by  the 
growth  of  the  pollen  tube  down  through  the  tissues  of  the 
style. 

A  question  of  much  interest  is  how  the  actual  meeting  of 
the  gametes  is  brought  about,  In  many  cases  it  is  un- 
doubtedly merely  by  chance;  the  random  swimming  of  the 
sperm  sooner  or  later  bringing  one  in  contact  with  an  egg. 
In  other  cases  the  movements  of  the  sperm  seem  to  indicate 
some  definite  attraction  by  the  egg.  It  has  been  shown, 
for  example,  that  the  sperm  of  some  Mosses  and  Ferns  are 
attracted  by  exceedingly  dilute  solutions  of  cane  sugar  and 
malic  acid  respectively,  traces  of  which  are  secreted  by  the 
tissues  in  the  vicinity  of  the  egg.  Also  the  sperm  of  some  of 
the  lower  animals  are  attracted  by  substances  eliminated  by 
the  egg  during  maturation.  In  such  instances  there  can  be 
but  little  doubt  that  chemical  stimulation  of  the  sperm  by 
specific  substances  plays  a  part  in  bringing  the  gametes  to- 
gether. This  is  an  example  of  CHEMOTAXIS:  a  phenomenon 
of  considerable  importance,  especially  in  the  behavior  of 
free-living  cells. 

Once  a  single  sperm  has  come  into  functional  contact  with 
the  egg,  it  initiates  a  chain  of  events  which  constitutes  fer- 
tilization. Although,  as  might  be  expected,  the  variations  in 
details  are  legion,  they  do  not  obscure  the  main  facts.  The 
first  reaction  on  the  part  of  the  egg  is  to  prevent  the  entrance 
of  other  sperm  and  thereby  to  insure  a  free  field  for  the  opera- 
tions of  the  first  arrival.  In  some  of  the  lower  plants  this  is 
accomplished  by  secreting  instantly  a  chemical  substance 
which  repels  other  sperm.  Frequently  among  animals  a 


ORIGIN   OF   THE    INDIVIDUAL  241 

jelly-like  layer  is  formed  about  the  egg,  or,  if  a  membrane 
is  already  present,  this  may  be  rendered  impermeable  or 
still  another  formed.  In  cases  where  the  egg  is  surrounded 
originally  by  a  dense  and  resistant  wall,  the  tiny  opening 
provided  for  the  entrance  of  the  sperm  is  closed.  How- 
ever, the  accessory  wrappings  about  certain  eggs,  such  as 
those  of  Birds,  have  no  relation  to  the  present  subject, 
since  they  are  secreted,  not  by  the  egg  itself,  but  by  glands 
in  the  wall  of  the  oviduct  some  time  after  fertilization  has 
occurred. 

The  reactions  of  the  egg  cytoplasm  that  exclude  accessory 
sperm  are  overshadowed  in  importance  by  others  which  upset 
the  stable  equilibrium  of  the  egg  and  render  its  surface 
permeable,  so  that  extensive  osmotic  interchanges  take  place 
between  the  cytoplasm  of  the  egg  and  its  surroundings.  Most 
often  this  is  visible  merely  in  a  shrinkage  of  the  cytoplasm 
due  to  loss  of  water,  but  sometimes  contractions,  amoeboid 
movements,  or  flowing  of  special  cytoplasmic  materials  to 
definite  regions  of  the  egg  are  visible.  In  any  event  it  is  cer- 
tain that  the  cytoplasm  undergoes  profound  changes  — 
its  organization  as  a  gamete  gives  place  to  a  reorganization 
which  establishes  and  determines  the  general  outlines  of  its 
subsequent  development  as  an  individual.  (Fig.  132,  A,  B.) 

Turning  now  to  the  nuclei,  known  as  male  and  female 
PRONUCLEI,  the  union  of  which  to  form  the  single  fertiliza- 
tion nucleus  (SYNKARYON)  is  the  climax  of  fertilization.  Dis- 
regarding the  flagellum  of  the  sperm,  which  disappears  as  it 
enters  the  egg,  we  find  that  the  sperm  nucleus  moves  through 
a  quite  definite  path  toward  the  center  of  the  egg  where  it  is 
met  by  the  egg  nucleus.  Both  the  pronuclei  now  become  re- 
solved into  chromosomes  which  lie  free  in  the  cytoplasm, 
while  a  pair  of  centrosomes,  surrounded  by  asters,  appear  and 
take  up  positions  on  either  side  of  the  chromosomes  to  form  a 


242  FOUNDATIONS   OF   BIOLOGY 

typical  mitotic  figure.  The  two  sets  of  chromosomes  form  an 
equatorial  plate  at  the  center  of  the  spindle,  thus  establishing 
at  once  not  only  the  mitotic  apparatus  for  the  first  division 
of  the  egg,  but  also  the  intimate  association  on  equal  terms 
of  chromosomes,  with  their  potentialities  from  the  two 
parents,  to  form  a  common  structure  —  the  nuclear  complex 
of  the  new  individual.  (Fig.  126,  I,  II.) 

Such  are  the  outstanding  facts  of  fertilization  which  a  host 
of  investigators  have  brought  to  light  chiefly  within  the  past 
century.  It  was  not  until  1839  that  Schwann,  with  the  es- 
tablishment of  the  'cell  theory/  recognized  the  egg  as  a  cell, 
and  sixteen  years  more  before  the  sperm  was  similarly  under- 
stood; while  the  first  realization  that  fertilization  is  an 
orderly  amalgamation  of  two  cells  to  form  one  came  during 
the  seventies  of  the  past  century.  Then  it  became  evident 
that  in  sexual  reproduction  each  individual  contributes  to 
the  formation  of  the  offspring  a  single  cell,  in  which  must  be 
sought  the  solution  of  the  problems  of  sex,  fertilization,  de- 
velopment, and  inheritance.  However,  the  concentration  of 
attention  on  the  cell  has  not  simplified  the  solution  of  these 
fundamental  problems;  but  rather  it  has  contributed  to  an 
ever-increasing  appreciation  of  the  complexities  of  cell  phe- 
nomena and  the  difficulties  of  formulating  them  in  general 
terms. 

With  a  realization  of  the  intricacies  of  the  phenomena 
involved  and  that  they  are  cell  phenomena,  we  may  turn  to 
a  consideration  of  the  significance  of  fertilization. 

D.   SIGNIFICANCE  OF  FERTILIZATION 

It  may  be  emphasized  again  that  fertilization  is  not  repro- 
duction.    Reproduction,  in  the  final  analysis,  is  division  - 
cell  division  or  the  detachment  of  a  portion  of  the  substance 
of  a  living  organism  to  constitute  another.     Rather  is  fer- 


ORIGIN   OF  THE   INDIVIDUAL  243 

tilization  a  phenomenon  associated  with  reproduction  —  so 
closely  associated  in  nearly  all  organisms  that  the  two  pro- 
cesses, evolving  together  from  relative  simplicity  to  great 
complexity,  have  reciprocally  influenced  one  another  until  in 
higher  forms  they  seem  to  be  related  as  cause  and  effect,  and 
reproduction  becomes  dependent  on  fertilization.  With  this 
somewhat  didactic  statement  of  our  viewpoint,  we  may  con- 
sider some  of  the  salient  features  of  the  almost  endless  discus- 
sion of  the  significance  of  fertilization. 

Quite  naturally  the  original  view,  emphasized  by  Harvey 
and  a  long  series  of  successors,  was  that  fertilization  funda- 
mentally is  a  reproductive  process,  and  echoes  of  this  idea 
are  preserved  in  certain  present-day  hypotheses,  on  the  basis 
of  such  facts  as  the  following.  The  mature  egg  pauses  in  de- 
velopment and  usually  comes  to  naught  unless  fertilized  — 
the  entrance  of  the  sperm  affording  a  necessary  stimulus  for 
the  resumption  of  cell  division  which  is  to  transform  the  egg 
into  the  adult. .  Again,  the  egg  typically  contains  only  half 
the  somatic  chromosome  complex  (simplex  group,  haploid 
number)  and  most  of  the  cytoplasm,  while  the  sperm  con- 
tributes a  reciprocal  haploid  set  of  chromosomes;  in  short, 
seemingly  transforms  what  is  essentially  a  half  into  a  whole. 

However,  it  does  not  necessarily  follow  from  these  facts 
that  fertilization  is  primarily  a  reproductive  process.  The 
evidence  against  this  conclusion  is  derived  largely  from  the 
relations  of  fertilization  and  reproduction  in  the  Protista,  and 
trom  the  development,  in  certain  cases,  of  eggs  without 
fertilization,  or  by  PARTHENOGENESIS.  A  single  example  of 
each  class  of  facts  will  suffice. 

1 .   Protista 

The  life  histories  of  nearly  all  Protozoa  and  Protophyta 
which  have  been  carefully  studied  include  a  period  in  which 


244  FOUNDATIONS   OF   BIOLOGY 

fertilization,  or  SYNGAMY,  occurs.  Under  usual  conditions, 
Paramecium,  for  instance,  reproduces  by  fission  two  or  three 
times  a  day  so  that  in  a  remarkabty  short  period  the  one  cell 
is  replaced  by  a  host  of  descendants.  Sooner  or  later,  how- 
ever, the  individuals  exhibit  a  tendency  to  unite  temporarily 
in  pairs,  or  CONJUGATE.  In  this  process  complicated  changes 
take  place  in  the  nuclei  of  the  cells,  during  which,  after  so- 
called  maturation  phenomena,  two  pronuclei  are  established 
in  each  individual  of  the  pair  of  conjugants.  Then  one  of  the 
pronuclei  in  each  conjugant  migrates  over  and  fuses  with  the 
stationary  pronucleus  of  the  other  to  form  a  synkaryon,  or 
fertilization  nucleus,  in  each  cell.  After  this  the  two  Para- 
mecia  separate,  reconstruct  their  characteristic  vegetative 
nuclear  apparatus,  and  proceed  to  reproduce  by  division  as 
before.  (Fig.  130.) 

This  is  fertilization  in  Paramecium,  and  it  is  generally  con- 
ceded that  the  primary  significance  of  synkaryon  formation 
must  be  sought  among  unicellular  forms  of  which  this  is  an 
example.  Accordingly  a  large  amount  of  experimental 
breeding  has  been  carried  out  on  Paramecium  and  its  allies. 
The  earlier  results  seemed  to  demonstrate  conclusively  that 
Paramecium  can  divide  only  a  limited  number  of  times,  say 
a  couple  of  hundred,  after  which  the  cells  die  from  exhaustion 
or  SENILE  DEGENERATION,  unless  conjugation  takes  place. 
In  other  words,  it  was  believed  that  periodic  REJUVENATION 
by  fertilization  is  a  necessity  for  the  continuance  of  the  life 
of  the  race.  And  therefore,  so  the  natural  conclusion  ran, 
protoplasm  is  unable  to  grow  indefinitely;  there  is  an  in- 
herent tendency  for  the  destructive  phases  of  metabolism 
to  gain  ascendency  over  the  constructive,  and  fertilization 
serves  to  maintain  or  restore  the  youthful  condition  and 
thus  secure  the  continuance  of  the  race. 

In  this  connection,  the  life  history  of  Paramecium  from  one 


ORIGIN    OF   THE    INDIVIDUAL 


245 


B 


1) 


G 


FIG.  130.  —  Diagram  of  the  nuclear  changes  during  conjugation  in  Paramecium 
aurelia.  A,  union  of  two  individuals  along  the  peristomal  region;  B,  degeneration  of 
rnacronucleus  and  first  division  of  the  micronuclei;  C,  second  division  of  micronuclei; 
D,  seven  of  the  eight  micronuclei  in  each  conjugant  degenerate  (indicated  by  circles) 
and  disappear;  E,  each  conjugant  with  a  single  remaining  micronucleus;  F,  this  nucleus 
divides  into  a  stationary  micronucleus  and  a  migratory  micronucleus  —  the  gametic 
or  pronuclei.  The  migratory  micronuclei  are  exchanged  by  the  conjugants  and  fuse 
with  the  respective  stationary  micronuclei  to  form  the  synkarya.  This  is  fertilization. 
G,  conjugants,  with  synkarya,  separate  (only  one  is  followed  from  this  point);  //,  first 
division  of  synkaryon  to  form  two  micronuclei;  I,  second  reconstruction  division; 
/,  transformation  of  two  micronuclei  into  macronuclei;  K.  division  of  micronuclei  ac- 
companied by  cell  division;  L,  typical  nuclear  condition  restored. 


246  FOUNDATIONS   OF   BIOLOGY 

conjugation  to  the  next  is  often  compared  to  the  life  of  a 
multicellular  organism  from  its  origin  as  the  fertilized  egg 
through  youth  and  adult  life  to  old  age.  The  striking  dif-r 
ference  is  that,  in  the  case  of  Paramecium,  the  products 
of  division  of  the  animal  which  has  conjugated  (EXCONJU- 
GANT)  separate  as  so  many  independent  cells,  all  of  which  are 
alike  and,  in  later  generations,  capable  of  conjugation;  while 
all  the  products  of  division  of  the  fertilized  egg  of  multi- 
cellular  forms  remain  together  as  a  unit  and  become  differ- 
entiated for  particular  functions  in  the  individual,  except  a 
few,  the  germ  cells,  which  retain  the  power  of  forming  new 
individuals.  Pushing  this  comparison  a  little  further,  if 
somewhat  fancifully,  it  is  stated  that  after  conjugation  in 
Paramecium  we  have  the  period  of  greatest  cell  vigor,  or 
youth,  followed  by  maturity  when  the  cells  are  ripe  for  con- 
jugation again,  and  in  the  absence  of  conjugation  —  and  only 
then  —  the  onset  of  old  age,  and  death.  Thus  death  has  no 
normal  place  in  the  life  history  of  Paramecium,  for  all  the  cells 
at  the  period  of  maturity  are  capable  of  conjugation.  On  the 
other  hand,  in  multicellular  forms  only  some  of  the  cells,  the 
germ  cells,  retain  this  power  —  the  somatic  cells  have  paid 
the  penalty  of  specialization  and  must  die.  Thus  death  of 
the  individual  except  by  accident  does  not  occur  among  uni- 
cellular forms  because  fertilization  'rejuvenates'  the  cell,  and 
the  cell  and  the  individual  are  one  and  the  same.  With  the 
origin  of  multicellular  forms,  involving  the  segregation  of 
soma  from  germ,  death  became  possible, and  was  established  — 
it  is  the  'price  paid  for  the  body.7  (Figs.  115,  135.) 

Suggestive  as  is  this  comparison  and  contrast  —  and  it  is 
not  without  some  justification  —  the  cardinal  fact  remains 
that  recent  work  has  demonstrated  that  Paramecium,  under 
favorable  environmental  conditions,  can  continue  reproduc- 
tion indefinitely;  at  least  for  fourteen  years  and  some  ten 


ORIGIN    OF   THE    INDIVIDUAL  247 

thousand  generations,  without  conjugation  and  without  any 
signs  of  degeneration.  In  other  words  fertilization  is  not  a 
necessary  antidote  for  inherent  senescence,  and  this,  taken 
in  connection  with  other  data  wrhich  point  in  the  same  direc- 
tion, such  as  the  fact  that  in  many  plants  sexual  propagation 
is  seldom  if  ever  resorted  to,  renders  it  fairly  safe  to  make  the 
general  statement  that  the  need  of  fertilization  is  not  a  pri- 
mary attribute  of  living  matter.  Now,  reproduction  is  such 
an  attribute  and  therefore  the  evidence  at  hand  indicates 
that  reproduction  and  fertilization  are  intrinsically  separate 
processes  which,  however,  have  become  closely  associated, 
especially  in  higher  forms. 

So  far  our  conclusion  is  entirely  negative  —  fertilization 
is  not  reproduction  and  is  not  intrinsically  necessary  for  re- 
production. What  then  is  its  significance?  Though  ferti- 
lization may  not  be  necessary  in  the  life  of  simple  organisms 
under  favorable  conditions,  this  does  not  indicate  that  it 
may  not  be  a  stimulus  to  protoplasmic  activity  when  it  does 
occur  —  perhaps  a  very  important  factor  under  special  en- 
vironmental conditions.  Indeed  there  is  no  doubt  that  con- 
jugation in  certain  cases  directly  results  in  stimulating  all  the 
vital  processes  of  the  cell  including  reproduction.  But  it 
would  seem  that  the  essential  factor  in  this  stimulation  is 
not  the  essence  of  fertilization  — •  synkaryon  formation.  In 
Paramecium,  for  example,  an  internal  nuclear  reorganization 
process  known  as  ENDOMIXIS  occurs  periodically.  Although 
endomixis  is  carried  on  by  each  cell  without  the  cooperation 
of  another  cell,  and  therefore  without  synkaryon  formation, 
nevertheless  it  apparently  effects  a  physiological  stimulation 
similar  to  that  which  follows  synkaryon  formation  during 
conjugation.  This  is  the  special  aspect  which  the  discussion 
of  the  'dynamic'  effect  of  fertilization  in  Protista  has  recently 
assumed.  (Fig.  131.) 


248 
A 

B 
C 
D 
E 
F 
G 
H 


FOUNDATIONS   OF   BIOLOGY 


FIG.  131.  —  Diagram  of  the  nuclear  changes  during  endomixis  in  Paramecium  aurelia. 
A,  typical  nuclear  condition;  B,  degeneration  of  macronucleus  and  first  division  of 
micronuclei;  C,  second  division  of  micronuclei;  D,  degeneration  of  six  of  the  eight 
micronuclei;  E,  division  of  the  cell;  F,  first  reconstruction  micronuclear  division: 
G,  second  reconstruction  micronuclear  division;  H,  transformation  of  two  micronuclei 
into  macronuclei ;  /,  micronuclear  and  cell  division;  J,  typical  nuclear  condition 
restored. 


ORIGIN    OF  THE   INDIVIDUAL  249 

2.  Metazoa 

Turning  from  Paramecium  and  its  allies,  we  may  consider 
some  evidence  among  higher  forms  in  regard  to  the  'dynamic' 
influence  of  fertilization.  Although  fertilization  is  usually 
necessary  for  the  resumption  of  the  series  of  cell  divisions 
which  paused  after  the  maturation  divisions,  and  which  are 
to  transform  egg  into  adult,  there  are  many  exceptional  but 
entirely  normal  cases  where  the  egg  proceeds  to  divide  of 
its  own  accord.  Such  parthenogenetic  eggs  are  formed  like 
other  eggs,  though  sometimes  without  synapsis  and  there- 
fore without  chromosome  reduction.  Thus  the  eggs  of  the 
Honey  Bee,  to  cite  the  most  interesting  case,  develop  either 
with  or  without  fertilization  —  fertilized  eggs  forming  fe- 
males and  unfertilized  eggs,  males.  Certain  species  of  Roti- 
fers and  Round  Worms  apparently  reproduce  solely  by 
parthenogenesis,  males  not  being  known.  Leaving  out  of  the 
question  the  effect  on  the  chromosome  complex,  it  is  at  once 
apparent  that  the  mere  fact  that  an  egg  divides  without  the 
influence  of  a  sperm  indicates  clearly  that,  in  such  cases  at 
least,  neither  structural  additions  nor  physiological  influences 
of  the  sperm  are  necessary  to  initiate  development. 

It  may  with  justice  be  urged,  however,  that  such  cases  of 
normal  parthenogenesis  are  special  adaptations  to  peculiar 
conditions  in  which  the  egg  has  usurped,  as  it  were,  the  usual 
sperm  function,  and  that  therefore  the  evidence  is  of  little 
weight  in  determining  the  primary  significance  of  fertiliza- 
tion. Accordingly  the  data  from  so-called  ARTIFICIAL 
PARTHENOGENESIS  are  particularly  cogent.  Within  recent 
years  it  has  been  found  that  the  eggs  of  a  considerable  num- 
ber of  Invertebrates  and  even  of  Vertebrates,  such  as  some 
Fishes  and  Frogs,  which  normally  require  fertilization,  can 
be  induced  to  start  development  'parthenogenetically'  by 


250  FOUNDATIONS   OF   BIOLOGY 

various  artificial  means  such  as  subjection  to  certain  chemi- 
cals, unusual  temperature  changes,  shaking,  or  the  prick  of  a 
needle  —  the  effective  stimulus  varying  with  different  species. 

Just  what  happens  in  the  egg  as  a  result  of  such  treatment 
is  open  to  discussion,  but  for  our  purposes  it  is  sufficient  to 
know  that  the  egg  begins  to  divide  in  normal  fashion.  This 
shows  conclusively  that  even  eggs  which  normally  require 
fertilization  are  intrinsically  self-sufficient  at  least  to  start  to 
develop,  and  therefore  this  strongly  indicates  that  an  inci- 
dental and  not  the  main  function  of  fertilization  is  to  stimu- 
late cell  division. 

Restating  the  evidence  in  its  bearings  on  the  meaning  of 
fertilization,  we  may  say  that  conjugation,  under  suitable 
environmental  conditions,  is  not  fundamentally  an  indis- 
pensable event  in  the  life  history  of  the  Protozoa,  and  further 
that  whatever  stimulus  is  associated  with  fertilization  is 
also  provided  by  endomixis  which  does  not  involve  synkaryon 
formation.  Similarly  in  the  Metazoa,  both  normal  and  arti- 
ficial parthenogenesis  indicate  that  the  egg  itself  comprises 
a  mechanism  which  is  capable  of  initiating  and  carrying  on 
development.  From  this  viewpoint,  fertilization  may  be 
satisfactorily  interpreted  as  a  means  of  insuring  under  special 
or  unfavorable  environmental  conditions  in  unicellular  or- 
ganisms, and  under  usual  conditions  in  the  eggs  of  multicellu- 
lar  forms,  a  suitable  stimulus  which  otherwise  might  be  un 
available  at  the  proper  time. 

Granting  then  that  one  aspect  of  fertilization  is  'dynamic,' 
what  is  its  main  significance?  Many  lines  of  evidence  at 
present  are  slowly  but  surely  converging  toward  the  view  that 
the  opportunities  which  fertilization  affords  for  changes  in 
the  complex  of  the  germ  are  of  paramount  importance. 
Fertilization  establishes  new  duplex  groups  of  hereditary 
characters  by  combining  diverse  simplex  groups  from  the 


ORIGIN   OF  THE   INDIVIDUAL  251 

two  gametes.  Careful  studies  show,  in  Paramecium  for  in- 
stance, that  variation  is  greater  after  than  before  fertilization, 
and  therefore  that  the  chief  significance  of  the  process  is  to 
afford  new  combinations,  some  of  which  will  more  effectually 
meet  —  be  better  adapted  to  —  the  exigencies  of  the  environ- 
ment, and  so  have  a  survival  value  for  the  organism  in  the 
struggle  for  existence.  So  whatever  the  primary  meaning  of 
fertilization  may  be,  its  importance  in  establishing  the  essen- 
tially dual  nature  of  every  sexually  produced  organism  is 
settled  beyond  dispute,  and  it  is  the  cardinal  fact  of  heredity. 
It  may  seem  strange  that  such  a  fundamental  phenomenon 
and  one  so  generally  distributed  throughout  the  animal  and 
vegetable  kingdom  should  so  long  have  eluded  solution.  The 
truth  probably  is  that  therein  lies  the  secret  of  the  difficulty. 
Whatever  fertilization  may  have  been  originally,  it  is  no 
longer  a  simple  process,  but  has  undergone  evolutionary 
specialization  hand  in  hand  with  that  of  other  functions  and 
with  the  structure  of  organisms.  To-day  one  or  another  of 
its  various  aspects  —  rejuvenation,  stimulus  to  development, 
control  of  variation,  or  basis  of  biparental  inheritance  — • 
may  assume  the  chief  role  or,  at  any  rate,  loom  largest  in  the 
mind  of  the  student.  The  popular  idea  that  fertilization  is 
reproduction  is  solely  due  to  the  fact  that  in  higher  organ- 
isms, if  fertilization  is  to  occur  at  all,  it  must  take  place  at 
that  period  in  the  life  history  when  the  individual  is  but  a 
single  cell  detached  from  the  parent  —  that  is,  at  repro- 
duction. 

E.   ORGANIZATION  OF  THE  ZYGOTE 

The  new  individual,  established  by  the  orderly  merging  of 
a  cell  detached  from  each  parent  in  sexually  reproducing 
species,  has  before  it  first  of  all  the  problem  of  assuming  the 
adult  form  by  a  complicated  developmental  process.  As  we 


252  FOUNDATIONS   OF   BIOLOGY 

have  shown,  this  involves  cleavage  of  the  egg,  followed,  in  the 
Metazoa,  by  blastula  and  gastrula  stages  during  which  the 
primary  germ  layers  are  established  —  the  fundament  out 
of  which  the  definitive  form,  organs,  and  organ  systems  of  the 
adult  are  evolved.  The  description  and  comparison  of  these 
processes  in  different  organisms  constitute  the  content  of  one 
aspect  of  EMBRYOLOGY.  We  must  be  satisfied  merely  with  the 
realization  of  the  fact  that  animal  development,  though  it 
varies  widely  in  producing  the  immensely  diverse  body  forms, 
exhibits  throughout  a  thread  of  similarity  in  its  broader 
fundamental  features.  (Figs.  19,  69.) 

But  embryology  is  something  more  than  the  description 
of  the  kaleidoscopic  series  of  stages  which  seem  to  melt  one 
into  the  other  as  development  progresses.  It  attempts,  espe- 
cially at  the  present  time,  to  look  below  and  beyond  structure 
to  the  processes  involved,  and  to  determine  how  the  sequence 
of  events  is  brought  about.  This  is  but  a  repetition  of  the 
stages  of  progress  in  all  science;  a  passage  from  the  descrip- 
tive to  the  experimental.  Although  many  of  the  results  thus 
far  secured  are  necessarily  largely  tentative,  they  have  gone 
far  toward  placing  the  science  of  biology  as  a  whole  on  an 
experimental  basis. 

From  what  the  pioneer  students  of  embryology  during  the 
seventeenth  and  eighteenth  centuries  saw,  or  thought  they 
saw,  with  simple  lens  and  newly  invented  compound  micro- 
scope, there  were  gradually  formulated  two  opposing  views 
of  development  which,  though  long  since  swept  aside  in  their 
original  form  as  a  result  of  the  increase  of  knowledge,  raised 
the  problem  of  problems  that  is  still  before  the  embryologist 
to-day.  In  brief,  one  view  virtually  denied  development  by 
maintaining  that  the  adult  organism  is  nearly  or  completely 
formed  within  the  germ,  either  in  the  egg  or  the  sperm,  which 
merely  by  expansion,  unfolding,  and  growth  gives  rise  to  the 


ORIGIN   OF  THE   INDIVIDUAL  253 

new  generation.  In  this  first  crude  form  the  PREFORMATION 
theory  demanded  the  'encasement'  of  all  future  generations 
one  within  another  in  the  germ  of  existing  organisms,  so  that 
when  it  was  computed  that  the  progenitor  of  the  human  race 
must  have  contained  some  two  hundred  million  homunculi 
(a  conservative  estimate,  to  say  the  least)  the  reductio  ad 
absurdum  was  irresistible. 

But  careful  studies  on  the  transformation  of  the  Hen's  egg 
into  the  chick  soon  made  it  clear  that  the  chick  is  not  pre- 
formed in  the  egg.  The  embryo  arises  by  a  gradual  process 
of  progressive  differentiation  from  an  apparently  simple 
fundament  —  it  is  a  true  process  of  development  or  EPI- 
GENESIS.  So  the  upholders  of  epigenesis  versus  preforma- 
tion  were  before  long  beyond  their  depth  and  in  danger  of 
attempting  to  get  something  out  of  nothing  —  lost  in  the 
miraculous! 

A  statement  in  such  succinct  form  tends  to  accentuate  the 
crudities  of  these  two  conflicting  views  —  "  pref ormation  ex- 
plaining development  by  denying  it  and  epigenesis  explaining 
development  by  reaffirming  it"  —  and  it  may  be  well  to  re- 
mark that  the  early  embryologists  with  the  means  at  their 
command  faced  a  stupendous  task  of  which  only  recent  work 
has  brought  a  full  appreciation. 

The  path  to  progress  cleared  by  the  realization  that  adult 
structures  are  not  preformed  as  such  in  the  egg,  and  that 
development  is  not  an  expansion  but  the  formation  —  the 
'becoming'  —  by  an  orderly  sequence  of  events  of  structures 
of  great  complexity  out  of  apparent  simplicity,  the  problem 
of  the  embryologist  was  to  determine  what  the  egg  structure 
is  and  how  related  to  that  of  the  adult.  To  trace  the  develop- 
ment of  these  studies  would  involve  the  history  of  embryology 
since  the  formulation  of  the  cell  theory.  We  must  confine 
ourselves  to  the  bare  statement  of  the  new  guise  in  which  the 


254  FOUNDATIONS   OF   BIOLOGY 

old  theories  of  preformation  and  epigenesis  confront  us  to- 
day as  a  result  of  recent  research. 

The  reader  already  recognizes  the  fertilized  egg  as  a  cell, 
with  its  nucleus  comprising  a  complex  of  quite  definite  ele- 
ments —  the  chromosomes  —  contributed  jointly  by  the  two 
gametes.  To  this  extent,  then,  the  nucleus  and  therefore  the 
egg  exhibits  a  ready-formed  structural  basis  which  (as  we 
have  alreadjr  suggested,  and  will  have  occasion  to  elaborate 
later)  seems  to  be  definitely  related  to  characters  which 
appear  in  the  offspring. 

Turning  to  the  egg  cytoplasm,  we  are  confronted  with 
conditions  which  are  not  so  uniform  but  nevertheless  highly 
suggestive.  In  the  first  place,  before  fertilization  the  egg 
possesses  a  definite  polarity,  expressed,  for  example,  in  the 
position  of  the  nucleus  and  the  distribution  of  food  material 
(yolk),  pigment  granules,  and  vacuoles.  This  polarity  is 
traceable,  in  part  at  least,  to  the  polarity  of  the  oogonia,  and 
through  them  to  the  germinal  epithelium.  In  brief,  the  egg 
as  a  whole  is  organized',  the  invisible  organization  of  the 
fundamental  matrix  of  the  cytoplasm  being  revealed,  in  part, 
by  the  disposition  of  various  elements  of  the  cell.  Now  in 
some  cases  this  cytoplasmic  organization  remains  essentially 
undisturbed  at  fertilization,  and  persists  as  that  of  the 
zygote,  while  in  others  it  is  superseded  sooner  or  later  by  a 
reorganization  which  establishes  that  of  the  new  organism. 
Herein,  apparently,  is  to  be  sought  the  explanation  of  the 
difference  in  behavior  —  in  potentialities  —  of  various  types 
of  eggs  during  cleavage  stages.  Clear-cut  examples  of  the 
two  chief  types  will  serve  to  bring  the  main  facts  before  us. 

The  first  type  is  well  illustrated  by  the  egg  of  a  Mollusc, 
Dentalium,  and  a  primitive  Chordate,  Cynthia.  The  egg  of 
the  latter  shows  at  the  first  division  five  clearly  differentiated 
cytoplasmic  regions.  For  the  sake  of  simplicity  these  may 


ORIGIN    OF  THE   INDIVIDUAL 


255 


G 


Fia.  132.  —  Development  of  a  Mollusc  (Dentalium) ,  after  removal  of  the  'polar 
lobe'.  A,  egg,  shortly  after  being  extruded  and  before  maturation  is  completed,  show- 
ing three  differentiated  regions.  B,  section  through  an  egg  after  fertilization,  showing 
cytoplasmic  rearrangement  involving  the  segregation  of  clear  '  polar  lobe '  at  p.  C,  nor- 
mal eight  cell  stage  with  'polar  lobe'  in  cell  D;  D,  normal  sixteen  cell  stage,  with  materi- 
als of  polar  lobe  now  in  cell  X;  E,  sixteen  cell  stage,  from  an  egg  with  the  'polar  lobe' 
removed  at  the  first  division;  F,  normal  larva  at  end  of  twenty-four  hours;  G,  larva 
(abnormal)  of  same  age  developed  from  egg  from  which  'polar  lobe'  was  removed; 
H,  normal  larva  of  seventy-two  hours.  /,  abnormal  larva  of  same  age  from'lobeless* 
egg.  (From  Kellicott,  after  Wilson.) 


256  FOUNDATIONS   OF   BIOLOGY 

be  described  as  hyaline,  light  and  dark  gray,  and  light  and 
dark  yellow.  As  cleavage  proceeds,  these  substances  are 
distributed  with  great  regularity  to  definite  cell  groups, 
which  in  turn  form  special  organs  or  organ  systems  of  the 
animal.  Thus  cells  which  receive  the  hyaline  region  form 
the  ectoderm;  those  which  receive  the  dark  gray,  the  endo- 
derm;  while  the  cells  with  light  or  dark  yellow  form  meso- 
dermal  structures,  'and  so  on.  And  further,  the  experimental 
removal  of  a  cell  or  cell  group  in  which  a  certain  substance 
is  segregated  results  in  an  embryo  deficient  in  the  very 
structures  which  this  normally  forms.  In  other  words,  the 
egg  cytoplasm  seems  to  be  a  mosaic  of  'organ-forming  sub- 
stances,' which  either  themselves  directly,  or  through  more 
fundamental  conditions  of  which  they  are  but  the  visible 
expression,  have  a  causal  relation  to  definite  adult  structures. 
Just  in  so  far  as  this  is  true,  the  adult  is  predelineated  in  bold 
lines,  though  not  actually  preformed,  in  the  egg.  (Fig.  132.) 
Passing  now  to  the  second  type,  represented  by  the  eggs  of 
Amphioxus  and  the  Sea  Urchins,  the  results  which  we  obtain 
seem  to  be  diametrically  opposite.  Although  in  the  egg 
of  the  Sea  Urchin  more  or  less  clearly  differentiated  cyto- 
plasmic  regions  appear  to  exist,  the  removal  of  a  part  of  the 
egg  before  division,  or  of  one  or  more  cells  during  cleavage, 
blastula,  or  gastrula  stages,  has  no  permanent  effect  on  the 
structural  integrity  of  the  developing  embryo.  Experi- 
ments show  that  each  of  the  cells,  even  as  late  as  the  sixteen- 
cell  stage,  has  the  power  to  develop  into  an  embryo  complete 
in  every  respect,  but  smaller  than  the  normal.  Or,  to  put  it 
another  way:  at  the  sixteen-cell  stage,  a  single  cell  which 
normally  forms,  let  us  say,  one-sixteenth  of  the  embryo,  if 
isolated  with  two  other  cells,  will  form  one  third  of  a  normal 
embryo;  if  isolated  with  three  other  cells,  will  form  one 
quarter;  and  so  on.  What  now  has  become  of  the  egg 


ORIGIN   OF  THE   INDIVIDUAL 


257 


organization?  Or,  if  we  lean  toward  a  mechanistic  inter- 
pretation of  development  or  life,  what  kind  of  a 'machine' 
is  it  which  has  such  potentialities?  (Fig.  133.) 

At  first  glance  the  behavior  of  these  two  classes  of  eggs 
seems  to  afford  results  which  are  irreconcilable  —  the  former 
A  B 


Fia.  133.  — •  Diagram  to  show  development  of  whole  eggs  and  isolated  cells  of  the 
two  cell  stage.  A,  Dentalium;  at  the  left,  development  of  the  whole  egg;  at  the  right, 
development  of  the  first  two  cells,  when  separated,  into  two  abnormal  larvae.  B,  Am- 
phioxus;  identical  experiment  at  the  two-cell  stage  resulting  in  two  perfect  small  larvae. 
(From  Wilson.) 

supporting  in  a  refined  form  the  perennial  doctrine  of  pre- 
formation,  and  the  latter  its  antithesis,  epigenesis.  But  an 
explanation  is  not  far  to  seek.  The  difference  apparently 
depends  upon  the  time  when  differentiation  of  the  egg  cyto- 
plasm is  chiefly  established.  If  this  occurs  before  or  at 
fertilization,  so  that  the  early  divisions  give  rise  to  dissimi- 


258  FOUNDATIONS   OF  BIOLOGY 

larly  organized  cells,  then  each  of  the  cells  is  not  equipotent 
and  the  mosaic  type  of  development  results;  but  if  the  initial 
differentiation  is  delayed  until  later,  or  is  relatively  slight  so 
that  the  cells  of  the  early  stages  are  all  essentially  similar, 
then  during  this  period  each  cell  is  totipotent —  the  whole 
forms  an  equipotential  system  —  as  exhibited  by  the  early 
stages  of  the  Sea  Urchin.  Thus  we  may  bring  under  one 
viewpoint  the  apparently  paradoxical  behavior  of  the  two 
classes  of  eggs,  for  it  turns  out  to  be  reducible  to  the  common 
factor,  differentiation.  In  one  case  this  has  progressed 
further  than  in  the  other  during  the  early  embryonic  stages. 
In  both  cases,  therefore,  development  is  epigenetic  in  its 
obvious  features.  (Fig.  134.) 

However,  since  cytoplasmic  differentiation  is  a  fact 
whether  it  appears  early  or  late,  we  have  merely  pushed  the 
solution  of  the  problem  further  back  and  the  question  be- 
comes: Is  there  a  primary  differentiation  and,  if  so,  where? 
It  is  not  possible  to  present  here  the  specific  evidence  on 
this  point,  but  the  reader's  knowledge  of  the  nucleus,  and 
particularly  its  definite  chromosomal  architecture,  will  lead 
him  to  anticipate  that  modern  research  tends  more  and  more 
to  emphasize  the  chromosome  as  representing  a  material 
configuration  —  a  packet  of  chemicals,  may  we  say  —  which 
is  transmitted,  in  a  way,  'preformed'  from  generation  to 
generation  and  determines  the  cytoplasmic  characteristics  of 
the  cells.  As  to  how  the  specific  physical  basis  of  inheritance, 
constituting  the  chromosomes,  is  related  to  cytoplasmic 
organization  and  to  characters  which  arise  later,  we  can  offer 
no  satisfactory  explanation  or  even  guess.  We  must  be 
content  with  a  discussion,  in  the  next  chapter,  of  some  of  the 
salient  facts  of  heredity  and  their  definite  association  with 
certain  chromosome  arrangements. 

But  in  so  far  as  the  nucleus  possesses  an  organization 


ORIGIN   OF  THE   INDIVIDUAL 


259 


which  is  definitely  related  to  differentiations  of  the  cyto- 
plasm, 'organ  forming  substances,'  or  characters  of  embryo 
and  adult,  we  may  look  upon  the  chromatin  to  this  extent  as 


D  E 

FIQ.  134.  —  Diagram  of  zones  of  cytoplasmic  differentiation  and  their  distribution 
at  the  first  division  of  the  egg.  A,  immature  egg,  assumed  to  have  no  definite  segrega- 
tion of  cytoplasmic  stuffs;  B,  mature  egg,  with  cytoplasmic  zones  established;  C,  first 
division  of  egg;  D  and  E,  two  types  of  two-cell  stages;  D,  Dentalium  or  Cynthia  type, 
with  one  cytoplasmic  zone  entirely  distributed  to  one  of  the  cells,  and  therefore  each  of 
the  two  cells,  if  separated,  gives  rise  to  an  abnormal  larva;  E,  Echinoderm  or  Amphioxus 
type,  with  equal  distribution  of  the  zones  to  both  cells,  and  therefore,  if  separated,  each 
of  the  two  cells  gives  rise  to  a  normal  larva.  (After  Wilson.) 

representing  a  sort  of  primary  preformation  which  is  real- 
ized by  a  process  of  building  up  —  epigenesis  —  as  one  char- 
acter after  another  becomes  established  in  the  development 
of  the  individual.  This  is  the  guise  in  which  the  old  problem 
of  preformation  versus  epigenesis  faces  the  biologist  to-day. 


260  FOUNDATIONS   OF   BIOLOGY 

The  early  embryologists  were  right  when,  watching  the  egg 
develop  into  the  chick,  they  maintained  that  development  is 
development  and  not  merely  an  unfolding  of  an  organism 
already  fashioned  in  more  or  less  definite  adult  form.  But 
it  took  two  centuries  of  research  to  reveal  the  fact  that, 
below  and  beyond  its  superficial  aspects,  there  is  a  germ  of 
truth  in  the  principle  of  preformation  hidden  in  the  nuclear 
architecture  —  that  the  origin  of  the  individual,  though 
obviously  through  epigenesis,  is  fundamentally  from  a  sort  of 
preformed  basis.  We  no  longer  bother  ourselves  with  the 
old  conundrum  as  to  which  is  more  complex,  the  hen  or  the 
egg,  but  recognize  the  fact  that  each  is  complex  in  its  way— 
the  simplicity  of  the  egg  being  more  apparent  than  real  as 
is  attested  by  every  endeavor  to  analyze  cytoplasm,  nucleus, 
chromosomes,  chromatin,  and  beyond. 


CHAPTER  XVII 

HERITAGE   OF   THE   INDIVIDUAL 

The  entire  organism  may  be  compared  to  a  web  of  which 
the  warp  is  derived  from  the  female  and  the  woof  from  the 
male.  —  Huxley. 

THE  old  adage  that '  like  begets  like '  expresses  the  general 
fact  of  HEREDITY.  Every  one  recognizes  that  parent  and 
offspring  agree  in  their  fundamental  characteristics  or  'be- 
long' to  the  same  'species.'  And  every  one  realizes  that  the 
resemblance  may  be  strikingly  exact  even  in  details  of  form 
or  behavior.  Family  traits  crop  out.  The  mere  statement 
of  striking  resemblances  among  the  individuals  of  a  family 
is  a  tacit  admission  that  no  two  individuals  are  exactly  alike; 
in  other  words  heredity  is  "organic  resemblance  based  on 
descent "  -  inheritance  of  the  characters  exhibited  by  the 
parents  is  not  complete,  there  is  VARIATION.  Indeed  "varia- 
tion is  the  most  invariable  thing  in  nature,"  but  one  must 
guard  against  the  impression  that  there  is  an  antithesis 
between  heredity  and  variation.  "Living  beings  do  not 
exhibit  unity  and  diversity,  but  unity  in  diversity.  In- 
heritance and  variation  are  not  two  things,  but  two  imperfect 
views  of  a  single  process." 

We  must  now  address  ourselves  to  the  problems  of  heredity 
and  variation  which  are  at  the  basis  not  only  of  what  organ- 
isms have  been  in  the  past  and  are  at  the  present,  but  also 
of  whatever  the  future  may  have  in  store  for  them.  Varia- 
tions are  the  raw  materials  of  evolutionary  progression  or 
regression.  From  a  broad  point  of  view,  the  origin  of 

261 


262  FOUNDATIONS    OF   BIOLOGY 

species  and  the  origin  of  individuals  are  essentially  the  same 
question.  If  we  can  solve  the  relations  of  parent  and  off- 
spring, the  origin  of  species  will  largely  take  care  of  itself.  As 
a  matter  of  fact,  historically  the  question  of  species  origin  was 
approached  first,  and  through  the  work  of  Darwin  became 
of  paramount  interest  in  the  latter  half  of  the  nineteenth 
century.  The  twentieth  century  finds  the  individual  —  the 
genetic  relation  of  parent  and  offspring  —  the  center  of 
investigation,  and  it  forms  the  science  of  genetics.  OR- 
GANIC EVOLUTION  established  the  general  fact  that  all  or- 
ganisms are  related  by  descent;  GENETICS  attempts  to  show 
how  specific  individuals  are  related. 

Even  further  has  the  pendulum  swung  from  the  general 
to  the  particular.  To-day  the  most  intense  investigation  is 
centered  not  on  the  heritage  of  the  individual  as  a  whole, 
but  on  particular  characters  of  the  individual.  The  concept 
has  arisen  from  recent  experimental  work  that,  for  practical 
purposes,  the  individual  may  be  regarded  as  congeries  of 
UNIT  CHARACTERS,  both  structural  and  physiological,  which 
are  more  or  less  stable,  and  which  are  inherited  as  units. 
But  the  analysis  does  not  stop  even  at  this  level.  There 
seems  to  be  good  reason  to  believe  that  each  so-called  unit 
character  is  represented  in  the  chromosomes  of  the  germ  ceils 
by  a  definite  factor,  determiner,  or,  as  it  is  now  usually 
termed,  GENE;  and  whether  or  not  a  given  character  will  be 
present  in  a  tree  or  a  man  depends  upon  whether  the  gene  for 
this  particular  character  entered  into  the  nuclear  complex  of 
the  fertilized  egg  which  formed  the  individual.  Therefore, 
geneticists  are  busy  plotting  the  relative  positions  which 
these  genes  occupy  on  certain  chromosomes  and  how  they 
may  ' cross-over'  from  one  chromosome  to  the  other  of  a 
synaptic  pair. 

Although  at  present  we  are  apparently  at  the  threshold  of 


HERITAGE    OF   THE    INDIVIDUAL  263 

great  advances,  in  knowledge  of  the  underlying  factors  of 
heredity,  the  data  already  accumulated  are  so  vast  that  we 
can  attempt  no  more  than  to  indicate  the  character  and 
promise  of  the  principles  already  discovered. 

We  may  survey  the  field  before  us  by  a  concrete  example. 
A  score  of  years  ago,  just  at  the  opening  of  the  modern  con- 
centrated attack  on  genetic  problems,  an  association  of  Brit- 
ish millers  awoke  to  the  fact  that  some  active  means  must  be 
taken  to  offset  the  increasingly  great  deficiency  in  quantity 
and  quality  of  the  wheat  yield.  Accordingly  they  com- 
missioned a  specially  trained  biologist  to  investigate  the  mat- 
ter. He  collected  many  different  varieties  of  domestic  and 
foreign  wheat,  each  known  to  have  one  or  more  good  qualities, 
and  studied  how  these  were  inherited.  Making  use  of  the 
data  thus  secured,  in  the  course  of  a  few  years  he  produced  a 
wheat  which  combined  the  good  qualities  of  several  varieties; 
including  high  content  of  gluten,  beardlessness,  immunity  to 
rust,  and  large  yield.  And  this  'made  to  order'  wheat  has 
proved  successful  in  the  British  Isles.  But  with  the  opening 
up  of  new  territory  in  western  Canada  another  obstacle  was 
encountered:  the  growing  season  was  too  short  for  the  finest 
varieties  of  wheat.  This  contingency  was  quickly  met  by 
transferring  the  quality  of  early  ripening  from  an  inferior 
grade  of  wheat  to  a  wheat  possessing  several  valuable  charac- 
ters. 

In  a  similar  fashion,  a  host  of  workers  have  performed  the 
impossible  of  a  few  years  ago.  Corn  of  desirable  percentage 
content  of  starch  or  sugar;  cotton  with  long  fibers  of  exotic 
varieties  and  quick  maturing  qualities  to  escape  insect 
ravages;  sheep  combining  choice  mutton  qualities  of  one 
breed  with  the  fine  wool  of  another  and  the  hornlessness  of 
a  third,  and  so  on  almost  ad  infinitum.  Furthermore,  there 
is  no  end  in  sight  of  the  new  stable  races  of  plants  and 


264  FOUNDATIONS    OF   BIOLOGY 

animals  which  are  forthcoming  as  the  principles  already 
known  are  applied,  and  subsidiary  ones  are  discovered.  And 
last  but  not  least,  Man  has  begun  to  study  himself  as  a  prod- 
uct of  breeding  and  the  process  of  evolution  —  to  determine 
the  distribution  of  characters  in  the  family,  and  the  conse- 
quences of  their  combinations  in  the  physical  and  mental 
make-up  of  the  individual. 

A.   HERITABILITY  OF  VARIATIONS 

What  then  are  the  basic  principles  of  heredity  which  are 
to-day  at  the  command  of  the  scientific  breeder?  To  answer 
this  question  it  is  necessary  to  go  into  some  details  because 
no  real  appreciation  of  the  underlying  principles  involved  is 
otherwise  forthcoming.  Most  of  these  details  have  been  ac- 
quired through  patient  investigations  made  from  the  standpoint 
of  so-called  pure  science  —  one  more  proof  of  the  indebtedness 
of  the  'practical  man  of  affairs'  to  the  biological  laboratory. 

In  the  Protista  the  problems  of  heredity  confront  us 
in  their  simplest,  though  by  no  means  simple,  form.  Para- 
mecium,  as  we  know,  divides  into  two  cells  which  through 
growth  and  reorganization  soon  are  to  all  intents  and 
purposes  replicas  of  the  parent  cell.  The  parent  has  merged 
its  individuality  into  that  of  its  offspring.  Thus  stated,  one 
does  not  wonder  that  parent  and  offspring  are  alike  —  each 
is  composed  of  essentially  the  same  protoplasm.  But  when 
we  come  to  multicellular  forms  in  which  reproduction  is 
restricted  to  special  germ  cells  which  involve  fertilization, 
confusion  is  apt  to  arise  unless  one  keeps  clearly  in  mind  - 
and  perhaps  exaggerates  for  the  sake  of  concreteness  —  the 
distinction  between  germ  and  soma  which  has  been 
previously  discussed.  Since  in  higher  forms,  to  which  brevity 
demands  that  our  attention  be  confined,  the  sole  connection 
between  parent  and  offspring  is  through  the  germ  cells,  it 


HERITAGE    OF  THE    INDIVIDUAL 


265 


follows  that  this  must  be  the  sole  path  of  inheritance.  In 
other  words,  whatever  characters  the  body  actually  inherits 
must  have  been  represented  by  genes  in  the  fertilized  egg 


Germplasm  \   Somatoplasm 


FIG.  135.  —  Scheme  to  illustrate  the  continuity  of  the  germplasm.  Each 
triangle  represents  an  individual  composed  of  qermplasm  (dotted)  and  somato- 
plasm  (clear).  The  beginning  of  the  life  cycle  of  each  individual  is  at  the 
apex  of  the  triangle  where  both  germplasm  and  somatoplasm  are  present.  In 
biparental  (sexual)  reproduction  the  germplasms  of  two  individuals  become 
associated  in  a  common  stream  which  is  the  germplasm  and  gives  rise  to  the 
somatoplasm  of  the  new  generation.  This  continuity  is  indicated  by  the  heavy 
broken  line  and  the  collateral  contributions  at  each  succeeding  generation  by 
light  broken  lines.  (From  Walter.) 

from  which  it  has  arisen:  and  conversely,  any  characters 
which  the  individual  can  transmit  must  be  represented  in 
its  germ  cells.  (Figs.  115,  135.) 

1  .   Modifications 

Every  individual  organism  —  a  man,  for  instance  —  is  a 
mosaic  not  only  of  inherited  characters  but  also  of  MODIFICA- 
TIONS of  the  soma  produced  by  external  conditions  during 


266  FOUNDATIONS   OF   BIOLOGY 

embryonic  development  or  later.  The  individual's  environ- 
ment, food,  friends,  enemies,  the  world  as  he  finds  it,  on  the 
one  hand,  and  on  the  other  his  education,  work,  and  general 
reactions  to  this  environment,  all  have  their  influence  on 
body  and  mind  arid  determine  to  a  considerable  extent  the 
realization  of  the  possibilities  derived  from  the  germ  — 
what  he  makes  of  his  endowment.  He  acquires,  let  us  say, 
the  strong  arm  of  the  blacksmith,  the  sensitive  fingers  of 
the  violinist,  or  the  command  of  higher  mathematics.  In 
other  words,  what  he  is  depends  on  his  heritage  and  what  he 
does  with  it.  Now,  if  he  does  develop  an  inherited  capacity, 
.can  he  transmit  to  his  offspring  this  talent  in  a  more  highly 
developed  form  than  he  himself  received  it?  Or,  must  his 
children  begin  at  the  same  rung  of  the  ladder  at  which  he 
started  and  make  their  own  way  in  the  world?  This  is  the 
old  question  of  the  inheritance  of  modifications,  or  so-called 
ACQUIRED  CHARACTERS.  Is  the  length  of  the*  Giraffe's  neck, 
to  take  a  classic  though  crude  example,  due  to  a  stretching 
toward  the  branches  of  trees  during  many  successive 
generations,  with  the  result  that  a  slight  increment  has  been 
gained  in  each  generation  and  inherited  by  the  following? 

We  cannot  enter  into  a  discussion  of  the  problem  here,  but 
must  simply  assure  the  reader  that  the  general  consensus  of 
opinion  of  biologists  is  certainly  to  the  effect  that  modifica- 
tions, or  changes  in  the  individual  body  due  to  nurture,  use 
and  disuse,  are  not  transmitted  as  such.  This  conclusion  is 
held  chiefly  because  there  is  no  positive  and  much  negative 
evidence  forthcoming,  and  also  because  there  is  no  known 
mechanism  by  which  a  specific  modification  of  the  soma  can 
so  influence  the  germ  complex  that  this  modification  will  be 
reproduced  as  such  or  in  any  representative  degree.  How- 
ever, it  should  be  emphasized  that  biologists  in  general  recog- 
nize the  potent  influence  of  environment  and  the  organisms' 


HERITAGE    OF   THE    INDIVIDUAL  267 

reactions  to  the  environment  on  the  destinies  of  the  race, 
even  though  they  see,  at  present,  no  grounds  for  a  belief  that 
any  specific  modification  can  enter  the  heritage  and  so  be 
reproduced. 

In  this  connection  the  question  of  the  inheritance  of  disease 
will  undoubtedly  arise  in  the  reader's  mind.  But  this  is  really 
not  a  special  case.  If  the  disease  is  the  result  of  a  defect 
in  the  germinal  constitution,  it  may  be  inherited  just  as  any 
other  character,  physiological  or  morphological.  But  if  the 
disease  is  a  disturbance  set  up  in  the  body  by  some  exigencies 
of  life  or  through  infections  by  specific  micro-organisms,  be- 
fore birth  or  later,  inheritance  does  not  occur;  though  it  is 
well  known  that  susceptibility  or  immunity  to  disease-pro- 
ducing organisms  —  the  'soil'  for  their  development  —  may  be 
inherited.  It  may,  however,  be  suggested  in  passing  that 
from  the  standpoint  of  the  individual  born  malformed,  struc- 
turally or  mentally,  as  a  result  of  parental  alcoholism  or  other 
obliquities,  it  probably  will  not  appear  of  the  first  moment 
that  the  sins  have  been  visited  otherwise  than  by  actual 
inheritance. 

The  whole  question  of  the  nonheritabilit}^  of  modifications 
or  acquired  characters  is  a  relatively  new  point  of  view  which 
has  been  fostered  by  an  ever-increasing  appreciation  of  the 
details  of  the  chromosome  mechanism  of  inheritance,  and  the 
realization  of  the  essential  truth  of  Weismann's  contrast  of 
the  soma  and  germ.  Indeed,  Lamarck  did  not  question  the 
inheritance  of  acquired  characters  and  made  it  the  corner- 
stone of  his  theory  of  evolution,  while  some  have  even  gone 
so  far  as  to  say  that  either  there  has  been  inheritance  of  ac- 
quired characters,  or  there  has  been  no  evolution.  But  the 
question  is  not  so  serious  as  that,  as  will  be  seen  later  on; 
though  it  obviously  is  profoundly  important  from  many 
viewpoints,  biological,  educational,  and  sociological. 


268  FOUNDATIONS   OF   BIOLOGY 

2.    Combinations 

Turning  from  modifications,  which  are  useless  to  the  geneti- 
cist, and  concentrating  attention  on  characters  which  repre- 
sent an  expression  of  germinal  factors,  we  see  that,  in  the 
final  analysis,  heredity  is  germinal  resemblance  among 
organisms  related  by  descent  —  a  consequence  of  the  con- 
tinuity of  cells  by  division/  Hereditary  differences  which 
appear  in  offspring  are  either  COMBINATIONS  of  ancestral  char- 
acters —  apparently  new  characters  which  owe  their  origin  to 
recombinations  of  the  germinal  factors  of  old  characters  —  or 
MUTATIONS  due  to  fundamental  changes  in  the  germinal  con- 
stitution, possibly  in  the  factors,  or  genes,  themselves.  ./ 

For  didactic  purposes  we  may  somewhat  arbitrarily  classify 
the  obvious  hereditary  differences  following  fertilization 
which  are  the  .result  of  recombinations  of  parental  characters 
represented  in  the  egg  and  sperm:  that  is,  cases  in  which 
nothing  is  apparent  which  is  not  clearly  related  to  the  condi- 
tions expressed  in  the  ascendants.  In  the  first  place  the  off- 
spring may  exhibit  a  character,  eye  color  let  us  say,  of  one 
parent  to  the  exclusion  of  that  of  the  other  —  the  character 
appearing  unmodified.  This  may  be  termed  ALTERNATIVE 
inheritance.  Or  the  offspring  may  seem  to  be  a  sort  of  mosaic 
of  the  characters  of  its  progenitors.  Here  each  parent  con- 
tributes a  certain  character  but  without  the  exclusion  of  that 
of  the  other  and  without  blending  —  the  offspring  exhibits 
MOSAIC  inheritance.  Sometimes  the  parental  traits  seem  to 
fuse  so  that  the  progeny  exhibit  a  more  or  less  intermediate 
and  different  condition,  as  in  the  color  of  the  skin  of  mulat- 
toes.  Such  a  result  is  known  as  BLENDING  inheritance.  Or 
again,  certain  characters  are  transmitted  from  males  solely 
to  female  offspring.  This  is  an  example  of  SEX-LINKED  in- 
heritance. In  still  other  instances  characters  of  grandparents 
which  are  invisible,  or  'latent/  in  the  parents  appear  again  in 


HERITAGE    OF   THE    INDIVIDUAL  269 

the  progeny.  This  has  long  been  known  as  ATAVISM.  Finally, 
characters  of  still  more  remote  ancestors  may  crop  out,  and 
constitute  REVERSIONS.  (Fig.  136.) 

3.   Mutations 

But  quite  different  results  now  and  then  occur.  Characters 
which  have  no  place  in  the  ancestry  appear  and  are  trans- 
mitted to  the  descendants.  Sometimes  these  new  inherited 
variations  are  only  slight  departures  from  the  parental  condi- 
tion, while  in  other  instances  they  are  quite  abrupt.  However, 
the  studies  of  deVries  and  others 
have  led  to  the  realization  that 
there  is  no  fundamental  difference 
between  the  two  classes  —  it  is 
chiefly  one  of  degree  — •  and  so 
we  speak  of  all  heritable  varia- 
tions, which  are  not  the  result  of 

FIG.  136.  —  Diagram  to  illustrate 
recombinations,       as       mutations,       three  types  of  inheritance  which  fol- 

and  contrast  combinations  and    ZZTZttXttZZ 

mutations  Sharply  With  modifica-      C<  blending.      (From  Conklin,  after 

Walter.) 

tions  which  are  not  transmitted  to 

the  offspring  and  are  the  results  of  environing  conditions  on 
the  soma  during  embryonic  development  or  later.  The  im- 
portance of  this  distinction  can  hardly  be  overemphasized 
because  it  makes  comprehensible  many  of  the  inconsistencies 
in  earlier  work  on  genetics,  as  will  immediately  appear. 

B.   GALTON'S  'LAWS' 

The  studies  of  Galton,  a  cousin  of  Darwin,  on  the  inheri- 
tance of  definite  characters  open  the  modern  era  of  scientific 
investigations  in  genetics.  In  particular,  his  work  on  the 
inheritance  of  characters  in  Man,  such  as  stature  and  intel- 
lectual capacity,  is  a  biological  classic  judged  by  the  momen- 
tous consequences  which  followed  from  the  discussion  it 


270 


FOUNDATIONS    OF   BIOLOGY 


evoked.  As  a  result  of  the  statistical  treatment  of  data, 
Galton  formulated  two  principles  of  heredity  which  may  be 
briefly  stated  as  follows: 

Law  of  Ancestral  Inheritance.    The  two  parents  contribute 
between  them,  on  the  average,  one  half  of  each  inherited 

Inches 
73  J 


72 


71 


70 


69 


68 


67 


64 


Mean  height  ofaU  parents          rjt       ' 


X 


FIG.  137.  —  Scheme  illustrating  Galton's  law  of  filial  regression,  as  shown 
in  the  stature  of  parents  and  children.  The  circles  represent  the  height  of 
graded  groups  of  parents  and  the  arrow  heads  show  the  average  heights  of 
their  children.  The  length  of  the  arrows  indicates  the  amount  of  '  regression ' 
toward  mediocrity.  (From  Walter.) 

faculty;  each  of  them  contributing  one  quarter  of  it.  The 
four  grandparents  contribute  between  them  one  quarter,  or 
each  of  them  one  sixteenth;  and  so  on. 

Law  of  Filial  Regression.  On  the  average  any  deviation 
of  the  parents  from  the  racial  type  is  transmitted  to  the 
progeny  in  a  diminished  degree;  the  deviation  from  the  racial 
mean  being  two  thirds  as  great  as  that  of  the  progenitors. 
(Fig.  137.) 


HERITAGE    OF   THE    INDIVIDUAL  271 

These  so-called  laws  taken  by  and  large  undoubtedly 
express  general  truths  —  offspring  inherit  much  more  from 
their  immediate  than  from  their  remote  ancestors;  and  off- 
spring of  gifted  or  deficient  parents,  judged  by  the  average 
standard  of  a  mixed  population,  regress  toward  mediocrity. 
But  the  '  laws '  are  not  particular^  helpful  in  arriving  at  the 
fundamental  principles  involved  in  heredity  because  the  data 
upon  which  they  are  founded  include  indiscriminately 
both  heritable  variations  and  modifications.  The  individ- 
ual's somatic  characters,  which  form  the  data,  belie  in  many 
cases  the  underlying  germinal  constitution  —  what  will  be 
transmitted  to  the  progeny.  Thus,  for  instance,  experiments 
show  that  when  the  germinal  make-up  of  all  the  mem- 
bers of  a  population  is  the  same,  the  regression  is  com- 
plete, no  matter  how  far  the  particular  parents  may  diverge 
somatically  from  the  population  average.  The  somatic 
divergence  represents  chiefly  modifications  which  are  not 
inherited.  Conversely,  when  the  divergence  of  the  parents 
from  the  population  average  is  due  to  characters  which 
represent  expressions  of  their  germinal  constitution,  then 
there  is  no  regression. 

C.   MENDELISM 

It  was  reserved  for  Mendel  to  apply  statistical  methods  to 
facts  observed  in  the  progeny  derived  from  carefully  con- 
trolled experiments  in  breeding.  In  other  words,  to  substi- 
tute for  'ancestral  generations,'  controlled  pedigrees  —  to  look 
forward  as  well  as  backward  and  thus  largely  to  remove 
the  unknown  and  unknowable  quantity  which  rendered  the 
materials  of  Galton  somewhat  delusive.  Mendel's  studies 
actually  were  made  a  score  of  years  before  Galton's,  but  failed 
to  reach  the  attention  of  the  biological  world  engrossed  in  the 
evolution  theory;  in  fact  were  never  known  to  Darwin  to 


272  FOUNDATIONS   OF   BIOLOGY 

whom  they  would  have  meant  so  much  in  his  work  to  secure 
experimental  data  in  heredity.  To-day  Mendelism  is 
essentially  a  science  in  itself,  with  its  own  vocabulary  of 
technical  terms.  We  can  attempt  no  more  than  to  make 
clear  its  fundamental  features  by  a  few  concrete  examples; 
the  first  from  Mendel's  own  work. 

Mendel  chose  seven  pairs  of  contrasting,  or  alternative, 
characters  which  he  found  were  constant  in  certain  varieties 
of  edible  Peas,  such  as  the  form  and  color  of  the  seeds,  whether 
round  or  wrinkled,  yellow  or  green;  and  the  length  of  the 
stem,  whether  dwarf  or  tall:  and  these  he  studied  in  the 
HYBRIDS.  One  ordinarily  thinks  of  a  hybrid  as  a  cross  be- 
tween two  species  or,  at  least,  two  characteristically  distinct 
varieties  of  animals  or  plants;  but  as  a  matter  of  fact  the  off- 
spring of  all  sexually  reproducing  organisms  are  really  hybrids 
because  two  parents  seldom,  if  ever,  are  exactly  the  same  in 
all  of  their  germinal  characters.  Consequently  the  offspring 
are  hybrids  with  respect  to  the  characters  in  which  the  par- 
ents differ. 

1.     Monohybrids 

Mendel  found,  for  example,  in  the  cross  between  the  tall 
and  dwarf  varieties  of  Peas,  that  all  of  the  progeny  in  the 
FIRST  FILIAL  (Fi)  generation  were  tall  like  one  parent,  there 
being  no  visible  evidence  of  their  actual  hybrid  character. 
Accordingly  tallness  was  designated  a  DOMINANT  (D)  and 
dwarfness  a  RECESSIVE  (d)  character.  His  next  step  was  to 
follow  the  behavior  of  these  characters  in  succeeding  genera- 
tions. Therefore  the  tall  hybrids  (Fi)  were  inbred  (self-fer- 
tilized) and  their  offspring,  the  SECOND  FILIAL  (F2)  generation, 
were  found  to  be  tall  and  dwarf  in  the  proportion  of  three  to 
one  (3D  :ld).  This  is  now  the  broadly  established  MENDELIAN 
RATIO.  Of  course  in  dealing  with  a  small  number  of  individ- 


HERITAGE    OF   THE    INDIVIDUAL  273 


138.  —  Inheritance  of  size  in  a  cross  between  a  tall  and  a  dwarf  race  of  garden  Peas. 
(After  Morgan.) 


274  FOUNDATIONS   OF   BIOLOGY 

uals  this  proportion  is  merely  approximate;  the  greater  the 
number  of  offspring,  the  closer  it  is  approached.  In  this  par- 
ticular case  Mendel  obtained  787  dominant  and  277  recessive 
individuals.  (Fig.  138.) 

Continuing  the  work,  Mendel  found  that  the  dwarfs  (reces- 
sives)  when  inbred  gave  only  recessives  generation  after 
generation,  and  accordingly  were  'pure',  or  EXTRACTED  RECES- 
SIVES. On  the  other  hand,  the  tall  plants  (dominants)  when 
inbred  proved  to  be  of  two  kinds,  one  third  pure  EXTRACTED 
DOMINANTS  which  bred  true  indefinitely,  and  two  thirds 
hybrids  like  their  parents,  giving  when  inbred  the  same  ratio 
of  three  dominants  to  one  recessive  in  the  THIRD  FILIAL  (F3) 
generation. 

Aside  from  his  masterly  foresight  in  realizing  that  success 
depended  on  simplifying  the  problem  by  dealing  with  definite 
contrasting  characters,  Mendel's  claim  to  fame  lies  chiefly 
in  his  discovery  of  a  simple  principle  by  which  the  results 
may  be  explained.  Since  the  hybrids  when  inbred  always 
give  rise  to  hybrids  and  also  to  each  of  the  parental  types  in  a 
pure  form,  it  must  be  that  the  factors  (genes)  which  deter- 
mine the  characters  in  question  are  SEGREGATED  in  the  germ 
cells.  That  is,  some  germ  cells  bear  one  gene  and  other 
germ  cells  the  other,  but  one  cell  never  bears  both.  If  we 
assume  that  the  germ  cells  contain  genes  which  determine 
the  size  of  the  plant  —  those  of  the  original  tall  parent  con- 
taining the  gene  for  tallness  (S) ,  and  those  of  the  dwarf  parent 
the  gene  for  dwarf  ness  (s)  —  then  the  hybrids  will  arise  from  a 
zygote  which  combines  both  genes  (Ss),  and  since  tallness  is 
dominant  over  dwarf  ness  all  will  be  tall.  Further,  when 
the  germ  cells  of  this  hybrid  (Ss)  mature,  if  these  genes 
segregate  so  that,  as  a  rule,  half  of  the  gametes  bear  S  and 
half  bear  s,  then  when  such  plants,  each  with  this  germinal 
constitution,  are  inbred  there  will  be  equal  chances  for 


HERITAGE    OF   THE    INDIVIDUAL 


275 


gametes  bearing  the  same  and  for  gametes  bearing  different 
genes  to  meet  in  fertilization. 

The  zygotes  are  1  SS :  2  Ss:  1  ss.  But,  since  S  is  dominant, 
the  resulting  organisms 
will  be  in  the  proportion 
of  3  tall  to  1  dwarf,  which 
is  the  familiar  3:  1  Men- 
delian  ratio  of  dominants 
to  recessives  in  the  F2 
generation.  The  import- 
ant point,  however,  is 
that  these  tall  organisms, 
although  they  all  appear 
alike  or,  as  we  now  say, 
belong  to  the  same 
PHENOTYPE,  are  different 
with  respect  to  their  germ- 
inal constitution;  because 
one  third  bear  germ  cells 
all  of  which  contain  the 
gene  S, .  and  two  thirds 
bear  germ  cells  half  of 
which  contain  S  and 
the  other  half  s.  Conse- 
quently the  phenotype  is 
composed  of  two  GENO- 
TYPES which  are  distin- 
guishable only  by  what 
they  produce.  (Fig.  139.) 

It  is  thus  apparent  why 
the  pure  tall  plants  (ex- 
tracted dominants)  al- 
ways breed  true,  and  why 


FIG.  139.  —  Diagram  of  a  Mendelian  mono- 
hybrid.  Results  of  crossing  large  size  (S)  and 
small  (s)  Pea  plants.  The  circles  represent  the 
zygotes  and  the  characters  of  the  soma  (pheno- 
type); the  letters  within  the  circles,  the  ger- 
minal constitution  (genotype).  The  letters  out- 
side the  recombination  square  represent  the  gam- 
etes. Note  that  each  of  the  parents  (P)  represents 
a  different  phenotype  and  genotype;  all  the  FI 
(one  shown)  belong  to  the  same  phenotype  and 
genotype ;  while  the  F2  represent  two  phenotypes 
and  three  genotypes.  The  relative  number  of 
individuals  composing  the  Fa  phenotypes  is  3 : 1 . 


276  FOUNDATIONS   OF   BIOLOGY 

the  pure  dwarfs  (extracted  recessives)  do  the  same  —  all  the 
germ  cells  of  one  bear  S  and  those  of  the  other, s.  The  plants 
are,  as  we  say,  HOMOZYGOUS  with  respect  to  the  characters  in 
question.  It  is  also,  clear  why  the  hybrids  give  rise  to 
hybrids  and  extracted  dominants  and  recessives  —  an  equal 
proportion  of  the  germ  cells  bear  S  and  s.  The  plants  are 

HETEROZYGOUS. 

The  real  difference  then  between  the  F2  hybrids  (Ss)  and 
the  extracted  dominants  (SS)  is  that  the  former  are  heterozy- 
gous and  the  latter  are  homozygous.  In  order  to  tell  which  is 
which,  since  they  are  phenotypically  the  same,  it  is  necessary 
to  breed  them.  When  self-fertilization  can  be  practiced,  as 
in  the  case  of  most  plants,  we  get  the  result  directly;  that  is 
an  individual's  progeny  are  either  all  dominants  or  dominants 
and  recessives  in  3  :  1  ratio,  and  thus  the  garnet ic  constitution 
of  the  parent  is  immediately  known.  However,  in  the  case 
of  animals,  where  self-fertilization  is  impossible,  the  deter- 
mination can  be  made  by  mating  the  dominants  with  reces- 
sives, for  a  homozygous  dominant  then  will  give  all  dominants 
while  a  heterozygous  dominant  will  give  half  dominants  and 
half  recessives.  Thus: 

Gametes  =  D\     /D          Dx    /d 

I  \/  I 


Gametes  dx    xd  dx   xd 

Possible  zygotes  =  100%  Dd         50%  Dd+50%  dd 

So  far  we  have  considered  the  inheritance  of  one  pair  of 
alternative  characters  —  the  resultant  of  a  pair  of  genes 
termed  ALLELOMORPHS  —  but  if  the  reader  has  grasped  the 
principles  involved,  we  may  pass  rapidly  over  cases  where 
two,  three,  or  more  pairs  are  concerned;  that  is  DIHYBRIDS, 

TRIHYBRIDS,  and  POLYHYBRIDS. 

2.   Dihybrids 
Mendel  found  the  solution  to  heredity  in  dihybrids  by 


HERITAGE    OF   THE    INDIVIDUAL 


277 


YR 


YR 


Yr 


yR 


yr 


FIG.  140.  — •  Diagram  of  a  Mendelian  dihybrid  — •  results  of  crossing  yellow 
round  seeded  (YR)  Peas  with  green  wrinkled  seeded  (yr).  The  circles 
represent  the  zygotes  and  the  characters  of  the  soma  (phenotype) ;  the  letters 
within  the  circles,  the  germinal  constitution  (genotype).  The  letter  groups  out- 
side the  recombination  square  represent  gametes.  The  hybrids  of  the  Ft 
generation  are  all  yellow  round  seeded  since  green  and  wrinkled  are  recessive. 
The  FI  plants  form  four  types  of  gametes  which  affords  sixteen  possible 
types  of  zygotes,  representing  four  phenotypes  (shown  graphically)  and  nine 
genotypes  (numbered).  There  is  one  pure  (extracted)  dominant  (1)  and  one 
pure  (extracted)  recessive  (9).  The  zygotes  numbered  4  are  identical  with  the 
FI  generation.  Four  are  homozygotes  (1,  7,  8,  9)  and  the  rest  are  heterozy- 
gotes.  The  relative  number  of  individuals  composing  the  phenotypes  is 
9:3  :3  :  1. 


278 


FOUNDATIONS   OF   BIOLOGY 


crossing,  for  example,  a  Pea  producing  yellow  round  seeds 
with  one  producing  green  wrinkled  seeds.     The  plants  in  the 


Key  to  Symbols 

•  =  Dark 
O    =  Light 
3    =  Curly 

•  =  Straight 


FIG.  141.  —  Scheme  to  illustrate  the  heredity  of  human  hair  characters.  Mendelian 
dihybrid.  Dark  and  curly,  dominant  characters;  light  and  straight,  recessive  charac- 
ters. The  arcs  represent  somatic  cells  of  four  individuals.  The  dominant  characters  are 
placed  on  the  outer  side  of  the  cells,  since  they  represent  the  visible  characters  (pheno- 
type).  The  gametes  are  placed  within  the  arcs  (cf.  Fig.  142).  (From  Walter.) 

FI  generations  bear  only  yellow  round  seeds,  and  therefore 
yellow  and  round  are  each  dominant  characters  when  paired 
with  green  and  wrinkled.  After  self-fertilization  such 
hybrid  plants  produce  offspring  (F2)  with  seeds  showing  all 
the  possible  combinations  of  the  four  characters,  and  in  the 


HERITAGE    OF   THE    INDIVIDUAL 


279 


proportion  of  9  yellow  round  to  3  yellow  wrinkled  to  3  green 
round  to  1  green  wrinkled.     (Fig.  140.) 

This  logically  can  only  be  interpreted  as  indicating  that 
one  of  the  original  parent  plants  bore  germ  cells  all  contain- 
ing the  genes  for  yellow  and  for  round  peas  (YR),  while  the 
other  parent  plant  bore  cells 
all  containing  the  genes  for 
green  and  for  wrinkled  (yr) . 
Such  being  the  case,  the  re- 
sulting zygote  is  YRyr,  and 
the  hybrid  which  it  forms 
develops  germ  cells  with  all 
the  possible  combinations 
of  these  genes  (except,  of 
course,  Rr  an.d  Yy)  which 
are  YR,  Yr,  yR,  and  yr. 
Now,  in  turn,  at  fertiliza- 
tion there  are  sixteen  possi- 
ble combinations  of  germ 
cells,  since  there  are  four 
different  kinds  of  sperm  and 
four  different  kinds  of  eggs 
with  respect  to  the  char- 
acters in  question.  Accord- 
ingly the  F2  generation, . 
which  is  produced  by  the  union  of  these  gametes,  is  repre- 
sented by  one  extracted  dominant  (YRYR),  one  extracted 
recessive  (yryr) ,  four  (including  the  former  two)  homozygotes 
and  twelve  heterozygotes.  These  sixteen  individuals  form 
nine  genotypes  but,  since  only  the  dominant  character  is 
expressed  when  dominant  and  recessive  genes  combine,  they 
are  resolvable  into  four  phenotypes  (YR,  Yr,  yR,  yr)  in  the 
ratio  9  YR  :  3  Yr  :  3  yR  :  1  yr.  Thus  the  9  :  3  :  3  :  1 


Number 
in  each 
class 

Genotype 

Phenotype 

Number 
in  each, 
class 

4 

fi^\ 

Dark  curly 

9 

2 

© 

2 

® 

1 

© 

1 

© 

Dark  straight 

3 

2 

© 

1 

® 

Light  curly 

3 

2 

/Os\ 

1 

<§) 

Light  straight 

1 

16 

16 

FIG.  142.  —  Diagram  classifying  the  six- 
teen possible  types  of  zygotes,  shown  in  the 
middle  of  Fig.  141,  according  to  genotypes 
(nine)  and  phenotypes  (four) .  (From  Wal- 
ter.) 


280  FOUNDATIONS   OF   BIOLOGY 

Mendelian  ratio  for  two  pairs  of  contrasting  characters  is 
merely  the  monohybrid  3  :  1  expanded.  Both  rest  on  the 
same  fundamental  assumption  that  there  is  an  independent 
assortment  of  the  genes  and  that  those  for  alternative  char- 
acters segregate  —  both  members  of  a  pair  of  allelomorphs 
can  never  occur  in  the  same  gamete.  (Figs.  141,  142.) 

3.    Trihybrids 

Similarly,  Mendelian  trihybrids,  for  example  the  cross 
between  tall  Peas  bearing  yellow  round  seeds  and  dwarfs 
bearing  green  wrinkled  seeds,  give  in  the  F2  generation  27 
genotypes  and  8  phenotypes;  the  relative  number  of  indi- 
viduals in  each  phenotype  being  in  the  proportion  27  :  9  :  9  : 
9:3:3:3:1.  Of  course,  in  nature  there  are  few  instances 
in  which  parents  and  offspring  differ  by  only  one,  two, or  three 
characters,  but  since  characters  arising  from  each  pair  of 
allelomorphs  can  usually  be  treated  singly,  expediency 
demands  that  the  analysis  be  made  with  respect  to  one 
or  two  pairs  at  a  time,  which  accordingly  is  the  usual 
method  of  procedure.  (Fig.  143.) 

4.    General  Principles 

Before  passing  to  certain  modifications  and  extensions  of 
Mendelian  principles,  it  may  serve  to  clarify  the  subject  if  we 
restate  in  slightly  different  form  and  then  summarize  the 
essential  facts  thus  far  discussed  on  the  basis  of  Mendel's 
own  work. 

Every  cell  of  the  soma  of  an  individual  bears  a  pair  of 
genes  for  each  'unit'  character  (e.g.,  size  in  the  case  of  the 
garden  Pea),  one  member  of  each  pair  having  been  derived 
from  each  gamete  which  contributed  to  the  individual's  make- 
up. When  both  genes  are  identical  (e.g.,  either  SS  or  ss)  they 
are  expressed  in  the  soma  (e.g.,  the  plant  is  tall  or  dwarf). 
The  individual  is  homozygous  with  respect  to  size.  But  when 


HERITAGE    OF   THE    INDIVIDUAL 


281 


SYR        sYR         SyR         syR        SYr 


sYr 


Syr 


syr 


SYR 


sYR 


SyR 


syR 


SYr 


eYr 


Syr 


syr 


FIG.  143.  —  Diagram  of  a  Mendelian  trihybrid.  Results  of  crossing  tall  Peas  bearing 
yellow  round  seeds  (SYR)  with  dwarf  Peas  bearing  green  wrinkled  seeds  (syr).  The 
circles  represent  the  zygotes  and  the  characters  of  the  soma  (phenotype) ;  the  letters 
within  the  circles,  the  germinal  constitution  (genotype).  The  letter  groups  outside  the 
recombination  square  represent  the  gametes.  The  Fi  hybrids  form  eight  types  of  game- 
tes, giving  sixty-four  possible  types  of  zygotes,  representing  eight  phenotypes  (shown 
graphically)  and  twenty-seven  genotypes.  There  is  one  pure  (extracted)  dominant 
(upper  left  corner)  and  one  recessive  (lower  right  corner).  Eight  are  homozygotes 
(diagonal  from  upper  left  to  lower  right  corner)  and  the  rest  are  heterozygotes.  The 
zygotes  in  the  diagonal  from  upper  right  to  lower  left  are  identical  with  the  ^i  generation. 
The  relative  number  of  individuals  composing  the  phenotypes  is27:9:9:9:3:3:3:l. 


282  FOUNDATIONS   OF   BIOLOGY 

the  two  genes  are  not  identical  (e.g.,  S  and  s),  then  one,  the 
dominant  (S),  is  expressed  in  the  soma  (the  plant  is  tall), 
while  the  other,  the  recessive  (s),  is  not  expressed.  The  indi- 
vidual is  heterozygous  with  respect  to  the  character  in  ques- 
tion (e.g.,  size). 

At  the  maturation  of  the  germ  cells  of  the  individual,  an  in- 
dependent assortment,  or  segregation,  of  the  genes  occurs  so 
that  the  gametes  bear  only  one  gene  (e.g.,  either  S  or  s)  for 
each  unit  character.  Thus  the  gametes  of  homozygous  indi- 
viduals are  all  alike  with  respect  to  the  gene  in  question  (e.g., 
all  bear  S  or  s) ,  while  the  gametes  of  heterozygous  individuals 
are  of  two  numerically  equal  classes  (e.g.,  half  bear  S  and  the 
other  half  bear  s). 

UNIT  CHARACTERS.  From  the  standpoint  of  heredity  an 
individual  organism  may  be  regarded  as  comprising  a  com- 
plex of  single  characters,  each  of  which,  broadly  speaking, 
behaves  essentially  as  a  unit. 

DOMINANCE.  When  the  determining  genes  (allelomorphs) 
for  each  of  a  pair  of  alternative  characters  are  present  in  the 
zygote,  one  (the  dominant)  is  expressed  in  the  resulting  indi- 
vidual; although  the  other  (the  recessive)  is  also  present  in  all 
of  its  somatic  and  in  one  half  of  its  mature  germ  cells.  In 
other  words,  the  recessive  is  not  expressed  unless  it  is  present 
in  duplicate. 

SEGREGATION.  The  genes  for  each  of  a  pair  of  alternative 
characters  are  never  both  present  in  the  same  gamete.  There- 
fore the  ripe  germ  cells  of  hybrids  fall  into  two  numerically 
equal  classes :  in  one  the  gene  of  the  dominant  character  and 
in  the  other  the  gene  of  the  recessive  character  is  segregated. 
This  is  the  so-called  purity  of  the  germ  cells. 

D.   NEO-MENDELISM 
It  so  happens  that,  as  data  accumulate,  it  becomes  more 


HERITAGE    OF   THE    INDIVIDUAL  283 

and  more  apparent  that  exceptions  which  prove  the  rule, 
make  it  necessary  to  revise  somewhat  our  ideas  regarding  the 
unity  of  unit  characters  and  the  dominance  of  dominants,  and 
to  accentuate  the  principle  of  segregation  as  the  prime  Men- 
delian  contribution.  A  few  examples  will  serve  to  bring  the 
main  facts  before  us. 

The  seven  pairs  of  contrasting  characters  in  Peas  which 
Mendel  studied  showed  essentially  complete  dominance  of 
one  character  in  each  pair,  and  therefore,  quite  naturally,  he 
laid  stress  on  this  principle.  As  a  matter  of  fact  we  may 
say  that  dominance  is  hardly  the  rule  because  there  are  in- 
numerable cases  in  which  the  hybrid  (Fi)  shows  a  different 
condition  from  either  of  the  parents.  For  instance,  on  cross- 
ing homozygous  red  and  white  races  of  the  Four-o'clock,  all 
the  progeny  in  the  heterozygous  (Fi)  generation  bear  pink 
flowers,  or,  we  may  say,  flowers  intermediate  in  color  between 
the  two  parents.  Neither  red  nor  white  is  dominant.  But  in- 
breeding these  give  an  F2  of  1  red,  2  pink,  and  1  white.  Thus 
the  typical  Mendelian  3  :  1  ratio  is,  so  to  speak,  automati- 
cally resolved  into  the  1:2:1  ratio  which,  when  one  character 
is  dominant,  is  only  patent  on  further  breeding.  (Fig.  144.) 

In  the  case  of  the  Four-o'clock,  only  the  hybrids  are  inter- 
mediate; segregation  occurs  as  usual  and  the  homozygous 
progeny  show  the  original  parental  characters  unmodified. 
But  sometimes,  with  the  apparent  lack  of  dominance,  segre- 
gation seems  not  to  take  place.  The  cross  between  white  and 
black  races  of  Man  is  a  typical  example. 

The  mulatto  (Fi)  is  intermediate  in  skin  color  between  the 
parental  types  and  even  in  the  F2  and  later  generations  rarely 
gives  pure  white  or  black  offspring.  But  an  adequate  Men- 
delian explanation  is  not  far  to  seek.  It  has  been  found  that 
both  white  and  black  are  really  composite  characters,  each 
made  up  of  varying  amounts  of  black,  yellow,  and  red  pig- 


284 


FOUNDATIONS    OF   BIOLOGY 


ments.  Now,  assuming  that  the  full-blooded  Negro  of  Africa 
bears  two  sets  of  genes  for  black  (AABB)  which  are  absent 
(aabb)  in  the  white  race;  then,  since  in  the  germ  cells  single 
genes  segregate,  the  cross  of  white  and  black  would  give  only 
a  single  set  of  genes  for  black  (AaBb)  and  the  hybrid  (Fi) 


Fi<3.  144.  —  Diagram  to  illustrate  the  results  from  crossing  white  and  red  flowered 
races  of  Four-o'clocks  (Mirabilis  jalapa).  The  somatic  condition  (phenotype)  is  shown 
graphically;  the  small  circles  represent  the  genes  which  are  involved. 

would  be  neither  black  nor  white,  but  intermediate.  Again, 
the  progeny  of  these  mulattoes,  that  is  the  F2  and  subsequent 
generations,  should  show  different  degrees  of  color,  as  they 
actually  do,  owing  to  varying  combinations  of  genes;  except 
in  the  small  number  of  cases  of  extracted  dominants  (black) 
and  extracted  recessives  (white).  Therefore  the  intermedi- 
ate color  of  the  offspring  of  black-white  crosses  is  reasonably 


HERITAGE    OF   THE    INDIVIDUAL 


285 


explained,  if  we  regard  the  character  black  as  the  expression 
of  at  least  two  pairs  of  genes,  neither  of  which  alone  gives 
black  but  only  when  reinforced  by  the  other.  The  infrequent 
appearance  of  pure  whites  or  blacks  in  the  F2  and  later  gen- 
erations is  not  due  to  lack  of  segregation,  but  to  the  fact  that, 
since  the  parental  characters  have  a  multiple  gene  basis,  the 
chances  are  slight  that  in  segregation  all  the  separate  genes 


A  B 


A  b 


a  B 


a  b 


A  B 


Ab 


a  B 


a  b 


A  B 

A  b 

a  B 

a  b 

A  B 

A  B 

A  B 

A  B 

A  B 

A  b 

a  B 

a  b 

A  b 

A  b 

A  b 

A  b 

A  B 

A  b 

a  B 

a  b 

a  B 

a  B 

a  B 

a  B 

A  B 

A  b 

a  B 

a  b 

a  b 

a  b 

a  b 

a  b 

FIG.  145.  —  Recombination  square  showing  the  result  of  mating  two 
mulattoes,  each  having  the  color  factors  AB  and  their  absence  ab  —  the  latter 
from  their  respective  white  parents.  The  color  of  the  offspring  varies  from 
black  (upper  left  corner)  to  white  (lower  right  corner).  Compare  Fig.  140. 
(After  Conklin.)  , 

will  be  brought  together  in  a  single  gamete  and  further 
that  such  a  gamete  at  fertilization  will  meet  one  similarly 
endowed.  (Fig.  145.) 

From  experiments  with  several  races  of  Locusts  which 
breed  true  for  color  pattern,  it  has  been  found  that  the  hy- 
brids between  any  two  show  the  entire  pattern  of  each  parent, 
one  superimposed  upon  the  other.  Thus,  again  merely  by 
inspection,  it  is  possible  to  determine  the  parental  compo- 
nents, and  since  such  hybrids  give  progeny  showing  the 
1:2:1  ratio,  it  is  evident  that  the  mosaic,  instead  of  blended, 
result  is  due  merely  to  the  fact  that  each  of  the  'alternative' 
characters  completely  expresses  itself. 


286  FOUNDATIONS   OF   BIOLOGY 

From  these  few  examples,  selected  almost  at  random  from 
the  wealth  of  data  at  hand,  it  is  clear  that  some  cases  of  blend- 
ing and  mosaic  inheritance,  as  well  as  alternative  inheritance, 
can  be  satisfactorily  interpreted  on  fundamental  Mendelian 
principles.  It  is  merely  necessary  to  bear  in  mind  that  when 
speaking  of  unit  characters,  we  mean  that  the  germinal 
physical  basis  of  characters,  that  is  the  genes  which  condition 
their  development,  behave  as  units,  for  now  we  know  that 
some  characters  are  determined  by  single  genes,  and  some  by 
multiple  genes.  And  further,  that  dominance  is  a  relation 
between  a  pair  of  genes  rather  than  between  their  expressions, 
characters.  Therefore  blending  inheritance  may  be  merely 
an  expression  of  the  action  of  several  pairs  of  genes,  each 
gene  displaying  dominance  for  one  member  of  a  pair;  while 
mosaic  inheritance  may  represent  the  extreme  where  each 
gene's  influence  is  exhibited  to  the  full  in  the  hybrid. 

Within  the  past  few  years  geneticists  have  been  able  by 
the  MULTIPLE  FACTOR  hypothesis  to  bring  into  line  with  the 
Mendelian  interpretation  the  inheritance  of  a  large  number 
of  characters,  especially  in  the  higher  animals.  Thus  stature, 
proportions  of  the  parts  of  the  body,  build,  as  well  as  nearly 
all  of  the  physiological  and  mental  characteristics  in  Man, 
are  evidently  dependent  upon  multiple  genes.  This  seems  so 
generally  true  in  the  higher  animals  and  plants  as  to  suggest 
that  their  characters  arc  genetically  relatively  complex  as 
compared  with  those  of  many  of  the  lower  organisms. 

So  it  happens,  as  is  usually  the  case,  the  more  a 
problem  is  studied  the  more  complex  it  appears  to  become. 
Suffice  it  to  say  that,  although  our  idea  of  'unit  characters/ 
'dominance/  and  even  'segregation'  is  to-day  somewhat 
broader  than  Mendel  conceived  on  the  basis  of  his  classic 
experiments,  it  is  evident  that  he  supplied  us  with  fundamen- 
tal principles  which  are  affording  a  common  denominator  for 


HERITAGE    OF   THE    INDIVIDUAL  287 

an  ever-increasing  number  of  facts  in  genetics.     Only  the 
future  can  determine  whether  they  are  universal. 

E.   MECHANISM  OF  MEND  ELIAN  INHERITANCE 

With  this  general  outline  of  the  Mendelian  principles 
before  us,  it  is  now  necessary  to  bring  them  into  relation  with 
the  facts  so  far  known  in  regard  to  the  structure  of  the  germ 
cells.  In  other  words,  we  have  assumed  germinal  factors, 
or  genes,  segregation,  etc.,  but  has  the  actual  study  of  cells 
(cytology)  given  any  evidence  of  the  physical  basis  of  genes 
and  of  a  segregating  mechanism?  The  reader  will  at  once 
answer  this  in  the  affirmative  on  the  basis  of  our  discussion 
of  the  origin  and  structure  of  the  germ  cells  and  their  behavior 
in  fertilization.  But  all,  or  nearly  all,  of  these  cardinal  facts 
were  unknown  when  Mendel  worked  and  this  makes  still 
more  remarkable  his  prevision  in  interpreting  his  results  in 
the  terms  he  did. 

The  essential  facts  may  now  be  restated  from  a  slightly 
different  viewpoint.  The  egg  and  sperm  each  carry  a  definite 
number  of  chromosomes  and  consequently  after  fertilization 
the  zygote  contains  a  double  set.  For  each  chromosome  con- 
tributed by  the  sperm  there  is  a  corresponding,  or  homolo- 
gous, chromosome  contributed  by  the  egg.  In  other  words, 
there  are  two  chromosomes  of  each  kind  which  may  be  con- 
sidered as  pairs.  When  division  of  the  zygote  takes  place  each 
chromosome  splits  into  two  chromosomes,  so  that  each 
daughter  cell  receives  a  daughter  chromosome  derived  from 
each  of  the  original  ones.  Since  all  the  cells  of  the  organism 
are  lineal  descendants  by  similar  mitotic  cell  divisions,  all  of 
its  cells  contain  the  double  set  of  chromosomes  —  half  paternal 
and  half  maternal;  and  since  the  primordial  germ  cells  have 
a  similar  origin,  they  also  have  a  double  set  of  chromosomes. 
But  during  the  maturation  process  synapsis  occurs:  that  is, 


288  FOUNDATIONS   OF   BIOLOGY 

homologous  chromosomes  of  paternal  and  maternal  origin 
unite  in  pairs  —  the  process  of  fertilization  which  gave  rise 
to  the  individual  being  consummated  in  the  ripening  of  its 
own  germ  cells.  But  this  union  is  only  temporary;  a  suc- 
ceeding mitosis,  instead  of  dividing  each  chromosome  as 
usual,  separates  the  maternal  and  paternal  chromosomes  of 
each  synaptic  pair  and  delivers  one  of  each  (though  rarely  all 
of  the  same  maternal  or  paternal  set)  to  the  two  arising  cells. 
Thus  each  mature  germ  cell  contains  one  member  of  every 
chromosome  pair  and  the  number  of  chromosomes  is  reduced 
one  half.1  (Fig.  146.) 

Mendel  postulated  that  the  genes  for  alternative  charac- 
ters segregate  in  the  formation  of  the  germ  cells  of  hybrids 
so  that  a  single  gamete  bears  one  and  not  both  genes  of  a 
pair  of  allelomorphs.  That  is  the  genes,  which  come  together 
in  the  zygote  which  forms  the  hybrid,  separate  again  in  the 
formation  of  its  own  germ  cells.  This  is  just  what  cytological 
studies  show.  Chromosome  behavior  exactly  parallels  the 
typical  behavior  of  the  Mendelian  gene,  because  in  the  matu- 
ration of  the  germ  cells  each  chromosome  of  paternal  origin 
separates  from  the  corresponding  chromosome  of  maternal 
origin.  The  genes  similarly  situated  on  homologous  paternal 
and  maternal  chromosomes  are  allelomorphs  and  are  segre- 
gated during  maturation.  And  further,  in  considering  Men- 
delian dihybrids  we  found,  for  instance,  that  genes  for  yellow 
and  round,  and  green  and  wrinkled  seeds  were  inherited  in  a 
fashion  which  indicated  that  yellow  and  round,  let  us  say,  are 
segregated  independently  of  each  other,  because  all  possible 
combinations  with  green  and  wrinkled  occur.  This  clearly  is 

1  It  will  be  recalled  that  in  plants  exhibiting  an  alternation  of  generations,  the  chromo- 
some reduction  occurs  at  the  formation  of  the  spores.  (Fig.  124.)  A  little  thought 
will  convince  the  reader  that  this  difference  is  of  no  importance  from  the  standpoint 
of  the  present  discussion,  because  we  are  interested  in  inheritance  from  sporophyte  to 
sporophyte  and  can  neglect  the  gametophyte  which  intervenes. 


HERITAGE    OF  THE    INDIVIDUAL  289 

oo 


ABCD 
0° 


JL 


aBCd. 


AbcD 
AaBbCcDd 

V 

FIG.  146.  —  Diagram  of  the  chromosome  cycle  of  an  animal.  Somatic  (diploid) 
chromosome  number  assumed  to  be  eight.  Paternal  chromosomes  (from  sperm)  = 
ABCD;  maternal  (from  egg)  =  abcd.  T,  union  of  nuclei  of  gametes,  each  with  a  simplex 
group  (haploid  number)  of  chromosomes,  in  the  zygote  at  fertilization  to  form  a  duplex 
group  (diploid  number)  of  chromosomes.  II,  III,  IV,  somatic  divisions  or  divisions  of 
germ  cells  before  maturation  (duplex  groups  of  chromosomes).  V,  synapsis,  involving 
pairing  of  homologous  paternal  and  maternal  chromosomes  to  give  the  haploid  num- 
ber of  paired  chromosomes.  VI,  reduction  division  —  separation  of  pairs  into  single 
chromosomes  again.  VII,  two  gametes,  with  simplex  groups  (haploid  number)  of  chro- 
mosomes; there  are  14  more  possible  combinations  of  the  chromosomes,  or  types  of 
gametes,  which  are  not  shown.  See  Fig.  147.  (After  Wilson,  slightly  modified.) 


290 


FOUNDATIONS   OF   BIOLOGY 


fully  accounted  for,  provided  the  gene  for  yellow  and  the  gene 
for  round  are  not  borne  by  the  same  chromosome,  since  in 
maturation  the  gametes  secure  one  of  each  pair  of  homologous 


FIG.  147.  —  Diagram  to  show  the  union  of  simplex  groups  of  either  the  chromosomes 
or  of  the  genes  of  the  gametes  to  form  the  duplex  condition  of  the  zygote  and  animal 
body;  and  then  their  pairing  at  synapsis,  and  segregation  in  the  gametes.  With  four 
pairs  of  chromosomes  or  of  genes  (Aa,  Bb,  Cc,  Dd)  there  are  sixteen  possible  types  of 
gametes.  (After  Wilson.) 

chromosomes  (a  simplex  group),  but  not  necessarily  all  of 
maternal  or  paternal  origin.     (Fig.  147.) 

In  short,  when  two  gametes  unite  they  each  contribute 
to  the  zygote  two  corresponding,  simplex  groups  of  genes  with 
the  result  that  the  offspring  is  of  a  double,  or  duplex  gene, 
constitution.  Similarly,  the  gametes  contribute  two  simplex 
chromosome  groups  so  that  the  zygote  is  of  a  duplex  chromo- 
some constitution.  Thus  both  the  chromosomes  and  the 


HERITAGE    OF  THE   INDIVIDUAL  291 

characters  (genes)  are  in  the  simplex  condition  in  the  gametes 
and  duplex  in  the  zygote.  This  close  parallelism  of  gene 
and  chromosome  behavior  affords  the  most  cogent  evidence 
that  the  chromosomes  supply  the  physical  basis  of  inheritance, 
and  that  Mendelian  segregation  and  related  phenomena  are 
facts.  For  all  practical  purposes  A,  B,  C,  D,  and  a,  b,  c,  d, 
in  figures  146  and  147  may  be  interpreted  either  as  chromo- 
somes or  as  characters. 

Turning  now  to  the  inheritance  of  characters  whose  genes 
are  borne  by  the  same  chromosome:  these  would  seem  to 
be  indissolvably  linked  together;  and  since  the  chromosome 
number  is  usually  not  large  —  there  are  twenty-three  or 
twenty-four  in  the  gametes  of  Man  —  compared  with  that  of 
heritable  characters,  we  would  expect  sometimes  to  find 
characters  linked  together.  That  is,  not  separately  in- 
herited as  are  yellow  and  round  in  our  example.  In  reality 
many  cases  are  known  in  which  characters  are  inherited 
in  groups.  The  inheritance  of  sex  and  sex-linked  characters 
will  make  the  main  point  clear,  and  at  the  same  time  serve 
to  bring  before  us  the  essential  facts  in  regard  to  the 
determination  of  sex. 

1 .   Sex  Determination 

The  reader  will  recall  that  in  the  general  description  of  cell 
structure  it  was  stated  that  every  cell  of  an  organism  contains 
a  definite  even  number  of  chromosomes.  As  a  matter  of  fact, 
in  most  instances  the  body  cells  of  one  sex,  usually  the  male, 
have  one  more  functional  chromosome  than  the  'regular'  set, 
and  therefore  an  odd  number.  This  extra  chromosome, 
which  is  commonly  designated  the  X,  or  SEX  CHROMOSOME, 
has  no  mate  at  synapsis,  remains  undivided  in  the  reducing 
maturation  division,  and  passes  entire  to  one  of  the  daughter 
cells.  Thus  two  classes  of  sperm  are  formed,  one  with  and 


292  FOUNDATIONS   OF   BIOLOGY 

the  other  without  the  X  chromosome  —  half  of  the  sperm 
contain  an  X  chromosome. 

Furthermore,  in  species  in  which  the  male  has  the  X 
chromosome,  the  female  has  two  of  them.  The  female 
therefore  has  one  more  chromosome  than  the  male.  Thus 
during  oogenesis  the  X  chromosomes  pair  in  synapsis  just 
as  the  other  homologous  chromosomes,  and  then  one  is  dis- 
tributed to  each  of  the  daughter  cells,  so  that  all  of  the  eggs 
contain  an  X  chromosome.  For  instance,  in  Man  the  somatic 
number  of  chromosomes  apparently  is  forty-seven  in  males, 
or  forty-six  plus  the  X  chromosome;  while  the  female  somatic 
number  is  forty-eight,  or  forty-six  plus  two  X  chromosomes. 
Half  of  the  sperm  contain  23  and  half  24  chromosomes;  all 
the  eggs  contain  24  chromosomes. 

Since  there  are  equal  numbers  of  sperm  with  and  without 
the  X  chromosome,  on  the  average  as  many  eggs  will  be 
fertilized  by  one  class  of  sperm  as  the  other,  with  the  result 
that  half  of  the  zygotes  will  contain  one  X  and  half  two  X 
chromosomes.  Obviously  the  former  will  develop  into 
males  and  the  latter  into  females,  since  the  somatic  cells 
of  males  have  the  X  chromosome  and  therefore  the  'sex 
gene'  in  simplex  condition,  and  similar  cells  of  females  have 
the  duplex  condition.  So  it  is  possible  —  it  has  been  accom- 
plished in  several  species  —  to  ascertain  the  sex  of  an 
embryo  by  counting  the  chromosomes  in  its  cells.  (Fig.  148.) 

Thus  there  is  good  cytological  evidence  that  sex  inheri- 
tance follows  the  Mendelian  formula.  The  male  carries  one 
sex  gene  (on  the  single  X  chromosome)  and  the  female  two 
sex  genes  (one  on  each  of  the  X  chromosomes) .  At  matura- 
tion these  segregate  so  that  the  male  is  heterozygous  and  the 
female  is  homozygous  in  regard  to  sex,  and  therefore  all 
possible  combinations  of  gametes  result  in  the  1  :  1  ratio  of 
males  to  females.  In  passing,  we  may  emphasize  that  this 


HERITAGE    OP   THE    INDIVIDUAL 


293 


shows  that  the  sex  of  an  individual  is  usually  determined 
at  the  time  of  fertilization,  and  not  subsequently  as  most  of 
the  well-known  theories  contend.  But  obviously  we  must 
guard  against  thinking  of  either  the  X  chromosome  or  the 
'sex  gene'  as  'producing'  sex.  Sex  is  a  complex  character 
whose  full  development  is  undoubtedly  conditioned  by  'sex 


MatureEgg  Oogonium 


sSpermatogon  ium 


FIG.  148.  —  Diagram  to  show  the  relation  of  the  two  classes  of  sperm  in  fertilization. 
The  formation  of  gametes  in  the  male  is  shown  at  the  left,  in  the  female  at  the  right; 
fertilization,  producing  the  male  or  female  zygote,  in  the  center.  X  chromosome  in 
black  (After  Wilson.) 

hormones/  etc.,  but  since  the  X  chromosome  is  the  differen- 
tial in  the  sexes,  it  is  to  that  extent  'sex-determining.' 

2.  Linkage 

Since  sex  is  regulated  by  an  internal  mechanism  which 
appears  to  be  the  same  as  that  which  determines  the  dis- 
tribution of  characters  in  Mendelian  inheritance,  it  might  be 
supposed  that  the  genes  of  other  characters  as  well  are 
carried  by  the  X  chromosome.  As  a  matter  of  fact  the 
behavior  in  inheritance  of  certain  characters  is  such  that  it 
can  only  reasonably  be  explained  on  this  assumption.  Ac- 
cordingly such  characters  are  known  as  SEX-LINKED.  This 


294 


FOUNDATIONS   OF   BIOLOGY 


brings  us  again  to  the  point  at  which  we  digressed  to  consider 
sex  —  the  discussion  of  genes  associated  on  the  same  chromo- 
some. One  example  must  suffice  to  bring  out  the  main 
facts. 

The  common  form  of  color-blindness  known  as  Daltonism, 
in  which  the  affected  individual  is  unable  to  distinguish  red 
from  green,  has  long  been  known  to  be  inheritable,  but  in  a 


F, 


xo 

d 


XX 

9 


K 


XO 


X 


XX 


XO 


XO 

d 


FIG.  149.  —  Diagram  to  show  the  inheritance  of  color-blindness  from  the  male. 
A  color-blind  male  (shown  in  black)  transmits  the  character  to  half  of  his  grandsons. 
•fc  indicates  the  'sex'  chromosome  with  the  gene  for  color-blindness.  (After  Morgan.) 

peculiar  crisscross  way.  The  condition  is  transmitted  from 
a  color-blind  man  through  his  daughters,  who  are  normal, 
to  half  of  his  grandsons;  and  from  a  color-blind  woman  to  all 
of  her  sons  and  none  of  her  daughters.  This  behavior  is 
readily  accounted  for  if  we  assume  that  the  gene  for  color- 
blindness is  associated,  when  present,  with  the  gene  for  sex 
on  the  X  chromosome,  and  that  color-blindness  develops  in 
males,  just  as  'maleness,'  when  it  is  simplex  or  from  one 
parent,  and  develops  in  females  when  it  is  duplex,  or  from 
both  parents.  (Figs.  149,  150.) 


HERITAGE    OF   THE    INDIVIDUAL  295 

Color-blindness  thus  serves  to  illustrate  the  association 
of  genes  of  different  characters  on  the  same  chromosome  and 
the  association  later  of  their  respective  characters  in  the 
adult.  But  the  presence  of  separate  genes  on  the  same 
chromosome  by  no  means  indicates  that  the  genes  must 
always  be  distributed  together,  for  there  is  considerable 
evidence  that  during  synapsis  genes  may  reciprocally  cross- 

XO  XX 


X    K9 

XX     XO 

9         d1 


HI 


XX     XX     XO     XO 

9          9         tf         d 

FIG.  150.  —  Diagram  to  show  the  inheritance  of  color-blindness  from  the  female. 
4  color-blind  female  (shown  in  black)  transmits  the  character  to  all  of  her  sons,  and  to 
half  of  her  grandsons,  and  to  half  of  her  granddaughters.  (After  Morgan.) 

over  from  one  synaptic  mate  to  the  other  and  thus  become 
separated  from  their  former  gene  associates  on  the  same 
chromosome.  This  CROSSING-OVER  removes  the  limitations 
which,  at  first  glance,  would  seern  to  confine  the  possible 
number  of  characters  capable  of  independent  segregation  in 
Mendelian  inheritance  to  that  of  the  chromosome  number, 
and  renders  invalid  any  objections  to  the  universality  of 
Mendelism  which  are  based  on  the  chromosome  mechanism 
as  at  present  understood.  And  further,  the  crossing-over 
gives  an  opportunity  to  determine  the  relative  positions  of 
different  genes  on  a  chromosome  —  if  it  is  assumed  that  the 


296 


FOUNDATIONS   OF   BIOLOGY 


distance  between  two  genes  is  proportional  to  the  percentage 
of  crossing-over  which  these  genes  show.     (Fig.  151.) 

F.  NATURE  versus  NURTURE 

From  one  viewpoint,  then,  the  individual  may  be  considered 
as  a  composite  of  very  many  unit  characters  which  behave  in  a 
definite  way  in  inheritance.  ''Expressed  otherwise,  and 


Ha 


JLc 


FIG.  151.  —  Diagram  to  show  a  possible  mechanism  of  crossing-over  during 
synapsis  of  homologous  paternal  and  maternal  chromosomes.  The  segments 
indicate  the  assumed  linear  arrangement  of  the  genes  with  allelomorphic  genes 
opposite  each  other.  I,  pair  of  chromosomes  which  have  entered  and  emerged 
from  the  synaptic  state  without  any  crossing-over;  I  la,  chromosomes  winding 
about  each  other  at  synapsis;  116,  separation  of  these  chromosomes,  involving 
breaking  at  the  points  of  crossing;  He,  their  emergence  from  synapsis  with  the 
members  of  the  pairs  of  allelomorphic  genes  interchanged.  (After  Wilson.) 

somewhat  fancifully,  individuals  are  simply  temporary 
kaleidoscopic  combinations  of  the  various  determiners  (genes) 
belonging  to  the  species;  the  act  of  reproduction,  especially 
the  reduction  division  and  subsequent  fusion,  providing  the 
new  turn  of  the  kaleidoscope."  But  since  the  life  of  an 
organism  is  one  continuous  series  of  reactions  with  its  sur- 
roundings, it  follows  that  nurture  plays  an  immensely  im- 
portant part  in  molding  the  individual  on  the  basis  of  its 
heritage.  This  is  especially  true  in  the  case  of  Man.  Devel- 
opment is  a  form  of  behavior,  and  how  a  child  develops 


HERITAGE    OF   THE    INDIVIDUAL  297 

physically  and  mentally  is  determined  not  by  its  heritage 
alone  nor  by  its  environing  conditions  alone,  but  by  both 
in  intricate  combination.  Although  apparently  we  do 
not  inherit  the  effects  on  our  forebears  of  their  surroundings 
and  training,  nevertheless  we  are  the  heirs  to  their  mores, 
which  entails  added  responsibilities  as  well  as  opportunities 
for  each  succeeding  generation.  Thus  'social  heredity'  bids 
fair  to  outstrip  our  conservative  and  essentially  unchanging 
inherited  nature.  The  EUTHENIST  emphasizes  nurture,  the 
EUGENIST  emphasizes  nature.  As  is  so  often  the  case,  how- 
ever, when  doctrines  are  opposed,  the  truth  combines  both; 
though  we  cannot  doubt,  knowing  what  we  know  of  the 
genetic  constitution  of  organisms,  that  from  the  standpoint 
of  permanent  advance  —  racial  rather  than  individual  - 
the  path  to  progress  is  through  EUGENICS,  the  science  of 
being  well  born.  "This  distinction  between  heritage  and 
acquirements  leaves  a  fatalistic  impression  in  many  minds, 
and  to  some  extent  this  is  justified.  We  cannot  get  away 
from  inheritance.  On  the  other  hand,  although  the  organism 
changes  slowly  in  its  heritable  organization,  it  is  very  modi- 
fiable individually;  and  this  is  Man's  particular  secret  —  to 
correct  his  internal  organic  inheritance  by  what  we  may  call 
his  external  heritage  of  material  and  spiritual  influences." 
(Thomson.)  (Fig.  152.) 

It  is  therefore  clear  that  the  problem  of  human  improve- 
ment has  two  aspects:  in  the  first  place,  the  effects  of  culture 
on  the  individual  which,  though  not  inherited,  are  cumulative 
from  generation  to  generation  through  training;  and  secondly, 
racial  betterment  through  breeding  the  best.  But  the 
reader  may  well  ask:  What  is  the  possibility  of  anything 
much  better  than  the  present  best  if  heredity  is  essentially  a 
recombination  of  the  characters  of  our  forebears  —  a  turn  of 
the  kaleidoscope? 


298  FOUNDATIONS    OF   BIOLOGY 

Although  we  are  wofully  ignorant  of  the  cause  of  variations, 
the  difficulty  is  more  apparent  than  real  and  arises  from  our 
absolute  ignorance  of  what  genes  really  are.  We  may 
conceive  them  to  be  chemical  molecules,  and  if  so  they  can 
change  only  by  an  alteration  of  their  chemical  constitution. 
And  for  all  we  know,  this  may  occur.  Or,  without  any 
change  in  the  genes  themselves,  their  expression  —  the 


\ 

\ 


\ 

\ 


\       *  /         V     \ 

\    «,  v  \  \ 


HERITAGE 

FIG.  152.  —  Scheme  to  illustrate  the  contributions  of  nature  and  nurture  to  the  make- 
up of  the  individual.  The  triangles  represent  various  types  of  individuals  which  may 
be  produced  by  the  same  germ  cells  (heritage)  if  environment  and  training  are  variable. 
The  foundation  of  the  "triangle  of  life"  is  heritage.  (After  Conklin.) 

chemical  effects  which  they  produce  —  may  change  by  the 
alteration  of  other  substances  with  which  they  react.  If  we 
interpret  such  phenomena  as  recombinations,  they  are 
profoundly  more  subtle  and  far-reaching  than  are  called  to 
mind  by  our  simile  of  a  kaleidoscope.  They  may  be  essen- 
tially infinite  in  number  and  infinite  in  potentialities  for  varia- 
tions in  the  germ  plasm  and  therefore  for  heritable  variations 
expressed  in  the  soma.  Again  it  is  possible,  perhaps  probable, 
that  inheritable  variations  are  often  the  result  of  chromosomes 
'accidentally'  losing  or  gaining  one  or  more  genes  during 


HERITAGE    OF   THE    INDIVIDUAL  299 

synapsis.  That  is,  one  member  of  a  pair  of  synaptic  mates 
leaves  with  the  other  member  certain  genes  for  which  it  gets 
none  in  return:  only  half  of  the  crossing-over  process  occurs. 
Such  a  phenomenon  would  probably  profoundly  modify  the 
constitution  of  both  chromosomes  involved  and  accordingly 
the  organisms  to  which  they  contribute.  And  all  such  types 
of  mutations  must  be  important  raw  materials  for  evolution. 

G.   SELECTION 

For  more  than  half  a  century  selection  has  been  something 
to  conjure  with  —  a  sort  of  creative  principle  to  explain  the 
progressive  changes  in  plants  and  animals,.  It  was  assumed 
that  the  SELECTION  of  a  certain  type  of  individual  for  breeding 
would  result  in  a  gradual  and  continuous  transformation  of 
the  race  or  species  in  the  direction  of  the  selection.  But 
Darwin  recognized  that  selection  in  itself  can  produce  nothing 
-  its  efficacy  depends  on  the  materials  afforded  by  variation. 
He  did  not  and,  in  fact,  could  not  make  the  modern  sharp 
distinction  between  modifications,  combinations,  and  muta- 
tions, but  accepted  all  variations  as  at  the  disposal  of  selec- 
tion. But  recent  work  indicates  that  selection  of  certain 
types  of  variations  effects  only  an  apparent  and  not  a  real 
change.  An  example  will  make  this  clear.  (Fig.  153.) 

Take,  say,  a  quart  of  beans  and  sort  them  into  groups  ac- 
cording to  the  weight  of  each  bean.  Then  put  each  group 
into  a  separate  cylinder  and  arrange  the  cylinders  in  a  series 
according  to  the  weight  of  the  enclosed  beans.  Now  if  we 
imagine  a  line  connecting  the  tops  of  the  bean  piles  in  each 
cylinder,  it  takes  the  form  of  a  typical  curve  of  probability,  or 
frequency  polygon.  A  similar  figure  would  be  obtained  by 
the  statistical  treatment  of  nearly  all  fluctuating  characters 
among  the  members  of  any  large  group  of  organisms,  or  of  the 
size  of  the  grains  in  a  handful  of  sand,  or  the  deviations  of 


300 


FOUNDATIONS   OF   BIOLOGY 


shots  from  the  bull's-eye  in  a  shooting  match.    Therefore  the 
variations  with  respect  to  a  given  character  very  closely  ap- 


Pure  Line 


population 


FIG.  153.  —  Diagram  to  illustrate  a  population  of  beans  and  its  five  compo- 
nent pure  lines.  The  beans  are  assorted  according  to  weight.  Tubes  containing 
beans  of  the  same  weight  are  placed  in  the  same  vertical  row.  The  population 
represents  the  quart  of  beans  discussed  in  the  text.  (From  Walter,  after 
Johannsen.) 

proximate  the  expectation  from  the  mathematical  theory  of 
probability,  or  chance,  and  the  reasonable  conclusion  is  that 
the  FLUCTUATIONS  are  a  resultant  of  a  large  number  of  factors 


HERITAGE    OF   THE    INDIVIDUAL 


301 


each  of  which  contributes  its  slight  and  variable  quota  to  the 
expression  in  a  given  individual.     (Figs.  154,  155.) 

The  question  is,  what  results  are  obtained  by  breeding  from 
individuals  which  exhibit  such  a  fluctuating  variation  to, 
let  us  say,  a  greater  degree  than  that  of  the  mean  of  a  mixed 
population?  The  reader  with  Galton's  theory  of  filial  regres- 


i 


\ 


A  B 

FIG.  154.  —  Model  to  illustrate  the  law  of  probability,  or  chance.  A,  shot  held  in 
the  funnel  at  the  top  of  the  board;  B,  the  shot,  released  by  opening  the  mouth  4F  the 
funnel,  have  fallen  through  the  series  of  hazards  (pins),  and  bejen  deflected  by  'chance' 
into  the  vertical  compartments  at  the  bottom.  The  curve  connecting  the  tops  of  the 
columns  of  shot  is  the  normal  probability,  or  frequency,  curve.  (After  Kellicott.) 

sion  in  mind  will  naturally  expect,  and  rightly,  that  the  off- 
spring usually  will  exhibit  the  character  to  a  less  degree  than 
the  parents  but  to  a  greater  degree  than  the  population.  The 
top  (mode)  of  the  curve  will  have  moved,  so  to  speak,  slightly 
in  the  direction  of  selection.  Now,  by  continuing  generation 
after  generation  to  select  as  parents  the  extreme  individuals, 
is  it  possible,  with  due  allowance  for  some  regression,  to  take 
one  step  after  another  indefinitely,  or  until  the  character  in 
question  is  expressed  to  a  degree  which  did  not  exist  previ- 


302 


FOUNDATIONS   OF   BIOLOGY 


ously?  The  experience  of  practical  breeders  gives  a  partial 
answer,  since  the  continual  selection  of  the  best  animals  for 
mating  and  the  best  plants  for  seed  has  been  a  profitable 
procedure.  But  it  has  long  been  known  that  after  a  certain 
amount  of  selection  has  been  practiced  it  ceases  to  be  so 


\ 


\ 


\ 


Inoktf    64     55    56    57    58     59     §0    61     62     63     64     65    66     67    68     69     10    7J 
Pergont      3       3       7      18     34    80    135  163   183^163  115    78    41  '  IB      6      5       & 


FIG.  155.  —  Normal  frequency  curve.  Plotted  measurements  of  the  height  of 
1,052  women.  The  height  of  each  rectangle  is  proportional  to  the  number  of  individuals 
of  each  given  height.  (Cf.  Figs.  153,  154.)  (From  Kellicott,  after  Pearson.) 

effective,  and  thenceforth  serves  chiefly  to  keep  the  character 
at  the  higher  level  attained.    (Fig.  156.) 

The  crux  of  the  matter  is  in  regard  to  exactly  what  the 
fluctuations  are.  Modifications  (non-heritable)  and  fre- 
quently combinations  (heritable)  give  a  normal  variability 
curve,  and  both  may  be  included  in  fluctuations.  This  mix- 
ture of  heritable  and  non-heritable  variations  is  what  makes 
confusion.  If  we  rule  out  combinations,  by  inbreeding 
or  by  self-fertilization  of  homozygous  individuals  —  establish 
PURE  LINES  —  then  the  fluctuations  are  all  modifications  and 
selection  is  ineffectual  with  characters  which  are  not  inherited. 


HERITAGE    OF  THE    INDIVIDUAL  303 

Pure  Lines 

The  importance  of  this  point  was  discovered  by  careful 
experiments  on  the  inheritance  of  characters  in  single  pure 
lines;  particularly  those  of  Johannsen  on  inheritance  in  a 
brown  variety  of  the  common  garden  Bean.  For  example,  by 
keeping  the  progeny  of  each  individual  bean  separate  from 
that  of  all  the  rest,  he  was  able  to  isolate  a  number  of  pure 
lines  which  differed  in  regard  to  the  average  weight  of  the 


FIQ.  156.  —  Schematic  representation  of  the  effect  of  selection  from  the  viewpoint 
of  Galton's  'law  of  filial  regression.'  (/)  Mode  before  selection;  2,  3,  4,  new  (successive) 
modes,  the  results  of  selections  of  individuals  at  #',  3',  4'-  The  mode  has  been  shifted 
in  the  direction  of  selection  (toward  the  right) .  But  there  has  been  each  time  an  amount 
of  regression  indicated  by  the  length  of  the  arrows. 

beans.  Selection  did  nothing  but  resolve  the  species,  or  the 
bean  'population'  with  which  he  began,  into  its  constituent 
'weight  types/  or  lines,  each  of  which  exhibited  a  characteris- 
tic variability  curve  of  its  own  with  a  mode  departing  more 
or  less  from  that  of  the  population.  But  when  Johannsen 
selected  within  a  pure  line  (ruled  out  combinations)  nothing 
at  all  resulted;  he  was  unable  to  shift  the  mode  because  he 
was  dealing  with  nonheritable  characters.  In  other  words, 
the  effect  of  selection  is  one  of  isolation  and  not  creation.  As 
a  rule  it  sorts  out  pre-existing  pure  lines  (lines  with  homo- 
geneous germinal  constitution)  from  a  population  and  then 
stops  —  though  if  selection  is  stopped  the  isolated  lines  usually 
soon  merge  again  into  the  original  population.  A  mutation 


304  FOUNDATIONS   OF   BIOLOGY 

must  occur  in  a  pure  line  for  selection  to  be  effective  —  and 
then,  ipso  facto,  the  single  pure  line  becomes  two.  (Fig.  153.) 

The  trend  of  present  work  certainly  seems  to  indicate 
that  these  conclusions  are  of  general  application  and  that  the 
explanation  of  the  long-accepted  feeling  that  selection  is 
'creative'  is  to  be  found  in  the  fact  that  variations  are  of 
three  sorts:  modifications  which  are  not  heritable  and  com- 
binations and  mutations  which  are  heritable.  Most  of  the 
variations  within  pure  lines  apparently  are  the  result  of  en- 
vironmental influences  recurrent  in  each  generation,  but  the 
germ  plasm  is  homogeneous.  The  variability  within  a  popu- 
lation is  the  composite  variability  of  its  component  pure  lines, 
but  the  germ  plasm  is  not  the  same  in  all  individuals  —  these 
may  be  segregated  into  groups,  the  pure  lines.  Thus,  very 
liberally  interpreted,  the  pure  line  concept  is  a  formal  expres- 
sion of  the  fact  that  most  of  the  variations  which  we  recog- 
nize are  either  somatic  or  the  result  of  recombinations  of 
diverse  parental  genes.  Accordingly  when  the  genes  of  the 
gametes  are  identical  (as  in  pure  lines)  the  latter  source  of 
variation  does  not  exist,  and  selection  is  powerless  except 
when  comparatively  rarely  mutations  occur.  (Fig.  157.) 

However,  some  recent  work  indicates  that  under  certain 
conditions  selection  appears  to  be  effective,  at  least  to  a 
limited  degree,  within  a  pure  line.  We  have  previously  seen 
that  certain  characters  are  the  expression  of  multiple  genes. 
In  some  such  cases  one  gene  is,  so  to  speak,  the  determining 
gene  for  the  character  as  a  whole,  while  associated  with  this 
gene  there  is  a  galaxy  of  modifying  genes  which  themselves 
do  not  find  expression  without  the  presence  of  the  determin- 
ing gene,  but  merely  serve  to  alter  the  character  expression 
of  the  latter.  Under  such  conditions  it  is  possible  to  modify 
the  character  by  selection  —  to  add  or  subtract  or  otherwise 
change  the  relationships  of  the  modifying  genes  to  the  pri- 


HERITAGE   OF  THE    INDIVIDUAL 


305 


mary  gene.  It  would  seem  however  that  the  effectiveness  of 
selection  of  this  sort  should  be  relatively  limited  in  any  par- 
ticular case,  and,  in  any  event,  the  data  thus  far  secured  do 


FiQ.  157.  —  Curves  illustrating  the  relation  between  pure  lines  and  popu- 
lations or  species.  A,  a  population  or  'species'  curve,  comprising  three  pure 
lines;  B,  the  separate  elements  (pure  lines)  of  A,  each  with  its  own  average 
and  variability.  (After  Kellicott.) 

not  fundamentally  alter  the  general  importance  of  the  pure 
line  concept. 

When  all  is  said,  it  is  clear  that  the  realization  of  certain 
categories  of  variations,  taken  in  connection  with  the  pure 
line  concept,  has  given  new  content  to  the  problem  of  selec- 
tion. The  appreciation  of  its  limitations  has  but  accentuated 
its  possibilities.  Selection  is  not  shorn  of  its  importance 
either  practical  or  theoretical.  Artificial  selection  is  useful  in 


306  FOUNDATIONS   OF   BIOLOGY 

separating  one  line  from  another,  as  is  attested  by  practical 
breeders  everywhere,  and  in  taking  advantage  of  mutations 
when  they  occur.  Most  of  the  'new  creations'  in  horticulture 
and  animal  breeding  are  the  result  of  hybridization  and  the 
rigid  selection  of  individuals  exhibiting  desirable  new  com- 
binations and  sometimes  mutations  which  hybridizing  seems 
to  induce.  Natural  selection,  in  a  quite  similar  manner,  may 
act  as  a  'sieve'  and  sort  out  new  combinations  and  mutations 
presented  —  leave  the  fit  and  eliminate  the  unfit  —  and  so 
afford  a  natural  explanation  of  the  adaptation  of  organisms 
to  their  environing  conditions.  (Fig.  194.) 

SUMMARY 

Before  leaving  the  subject  a  brief  summary  of  the  most 
important  general  principles  which  the  study  of  genetics  has 
thus  far  afforded  may  be  helpful.  In  the  first  place,  it  appears 
clear  that  the  basis  of  inheritance  is  in  the  germinal  rather 
than  in  the  somatic  constitution  of  the  individual.  A  charac- 
ter to  be  inherited  must  be  innate  in  the  germ  cells,  and  there 
is  no  satisfactory  evidence  that  modifications  of  the  body, 
'acquired  characters,'  can  be  transferred  to  the  germ  and  so 
inherited.  Secondly,  characters  or  groups  of  characters  are 
usually,  if  not  universally,  inherited  as  definite  units.  These 
follow  Mendelian  principles  of  segregation  and  recombination 
in  the  formation  of  the  germ  cells  of  an  individual,  so  that 
paternal  and  maternal  contributions  are  readjusted  in  all  the 
combinations  which  are  mathematically  possible.  And 
finally,  the  germinal  factor  basis  (genes)  of  unit  characters  is 
remarkably  constant.  Selection  is  apparently  powerless  to 
alter  it,  but  merely  sorts  out  what  is  already  there,  or,  taking 
advantage  of  such  changes  (mutations)  as  do  occur,  deter- 
mines their  survival  value  for  their  possessor  in  the  struggle 
for  existence. 


CHAPTER  XVIII 
ADAPTATION  OF  ORGANISMS 

Every  creature  is  a  bundle  of  adaptations.     Indeed,  when 
we  take  away  the  adaptations,  what  have  we  left? 

— Thomson  and  Geddes. 

ORGANISMS  are  systems  dependent  for  their  maintenance 
and  operation  upon  energy  liberated  by  chemico-physical 
processes  in  protoplasm,  and  therefore  any  and  all  influences 
which  induce  changes  in  the  structure  or  functions  of  an 
organism  must  initially  modify  the  underlying  phenomena 
which  are  responsible  therefor.  In  a  word,  organic  response 
is  a  problem  of  metabolism.  Although  it  is  highly  important 
that  this  cardinal  fact  be  clearly  grasped,  the  science  of 
biology  to-day  is  not  in  a  position  to  interpret  the  responses 
of  organisms  in  these  fundamental  terms,  and  we  shall  merely 
present  some  representative  instances  to  illustrate  the  fact 
that  the  response  of  organisms,  as  exhibited  in  active  adjust- 
ment —  adaptation  —  of  internal  and  external  relations, 
overshadows  in  uniqueness  all  other  characteristics  of  life 
and  at  one  stroke  differentiates  even  the  simplest  organism 
from  the  inorganic. 

Overwhelmingly  striking  as  is  the  fitness  of  organisms  to 
their  physical  surroundings,  we  must  not  lose  sight  of  the  fact 
that  the  environment  itself  presents  a  reciprocal  fitness.  This 
results  from  the  "unique  or  nearly  unique  properties  of  water, 
carbonic  acid,  the  compounds  of  carbon,  hydrogen,  and  oxy- 
gen. ...  No  other  environment  consisting  of  primary  con- 
stituents made  up  of  other  known  elements,  or  lacking  water 

307 


308  FOUNDATIONS   OF  BIOLOGY 

and  carbonic  acid,  could  possess  a  like  number  of  fit  charac- 
teristics, or  in  any  manner  such  great  fitness  to  promote  com- 
plexity, durability,  and  active  metabolism  in  the  organic 
mechanism  which  we  call  life."  (Henderson.) 

A.   ADAPTATIONS  TO  THE  PHYSICAL  ENVIRONMENT 

In  any  consideration  of  the  reciprocal  relations  which  must 
exist  between  organisms  and  their  surroundings,  of  first  im- 
portance is  the  inconstancy~of  the  latter.  Uncertainty  is  the 
one  certainty  in  nature  and  accordingly  the  response  of  living 
things  —  their  adaptability  to  environmental  exigencies  — 
is  at  once  the  most  striking  and  indispensable  adaptation. 

1.   Adaptations  Essentially  Functional 

Although  the  changes  of  the  environment  are  almost  in- 
conceivably complex  —  witness  the  kaleidoscopic  series  of 
events  exhibited  in  the  hay  infusion  microcosm  —  there  are 
certain  general  conditions  which  every  environment  must 
supply,  and  without  which  life  cannot  exist.  These  are  food, 
including  water  and  oxygen,  certain  limits  of  temperature 
and  pressure. 

FOOD.  As  we  know,  food  represents  the  stream  of  matter 
and  energy  which  is  demanded  for  the  metabolic  processes 
of  living  matter.  And  each  and  every  element  which  forms 
an  integral  part  of  protoplasm  must  be  available.  Since 
all  protoplasm  consists  chiefly  of  a  dozen  chemical  elements, 
these,  of  course,  must  be  present;  and  further,  since  proto- 
plasm is  a  colloidal  complex  in  which  water  plays  a  funda- 
mental role,  life  processes  without  water  are  impossible. 
But  the  old  adage  that  what  is  food  for  one  is  another's 
poison  has  a  broader  content  than  is  immediately  apparent. 
Although  it  is  true  there  are  general  'food-elements'  which 
all  life  demands,  it  is  equally  true  that  the  combinations  in 


ADAPTATION    OF   ORGANISMS 


309 


which  these  elements  must  be  presented  to  the  organism,  in 
in  order  to  be  available  for  its  metabolic  processes,  are  sub- 
ject to  the  widest  variation. 

We  have  emphasized  and  contrasted  the  nutrition  of  a 
typical  animal,  green  plant,  and  colorless  plant,  and  have 
seen  the  reciprocal  part  which  they  play  in  the  circulation 
of  the  elements  in  nature,  so  it  is  only  necessary,  with  these 
facts  in  mind,  to  cite  special  cases  in  order  to  illustrate  the 
adaptation  of  special  groups  of  organisms  to  special  condi- 


FIG.   158.  —  Portion  of    filaments  of   Begyiotoa   alba    (a),  and   two  cells  of  Beggiotoa 
mirabilis  (6)  showing  enclosed  sulfur  granules.     (From  Buchanan.) 

tions  of  existence.  The  demands  of  the  so-called  Sulfur 
Bacteria  and  the  Yeasts  are  in  point. 

The  Sulfur  Bacteria  (Beggiotoa)  live  in  water  containing 
sulfuretted  hydrogen,  from  which,  by  oxidation,  they  obtain 
energy  and  store  up  within  the  protoplasm  free  sulfur  in  the 
form  of  tiny  granules.  And  then  by  further  oxidation  they 
transform  the  sulfur  into  sulfuric  acid  and  excrete  it.  Thus 
a  gas  which  is  poisonous  to  nearly  all  organisms  is  for  Beg- 
giotoa a  necessary  life  condition.  (Fig.  158.) 

The  Yeasts  include  a  host  of  microscopic  colorless  plants 
which  play  an  important  part  in  the  simplification  of  organic 
compounds.  (Fig.  159.)  Being  devoid  of  chlorophyll, 
Yeasts  of  course  lack  photosynthetic  powers,  though  like 
many  other  colorless  plants  they  are  not  dependent  upon 


310  FOUNDATIONS   OF   BIOLOGY 

proteins  for  nitrogen  but  obtain  it  in  less  complex  forms. 
But  the  essential  fact  of  interest  at  present  is  the  chemical 
changes  associated  with  Yeast  metabolism  —  the  transforma- 
tion of  a  large  proportion  of  the  sugar  content  of  the  medium 
in  which  they  live  into  alcohol  and  carbon  dioxide.  This 
process  of  alcoholic  fermentation  may  be  approximately 
expressed  by  the  formula: 

C6H12O6(sugar)+yeast=2  C2H5OH (alcohol) +2  CO2 
The  explanation  is  not  far  to  seek.     Deprived  of  an  adequate 
supply  of  air,  Yeasts  resort  to  the  energy  released  when,  with 
the  decomposition  of  the  sugar,  the  carbon  and  oxygen  unite 


FIG.  159.  —  Yeast  cells,  very  highly  magnified.  A,  cell  showing  granular 
cytoplasm  and  a  large  vacuole;  B,  showing  nucleus;  C,  cell  budding; 
D,  mother  cell  and  bud  after  division  is  completed. 

as  C02.  The  formation  of  alcohol  by  the  remnants  of  the 
sugar  molecules  is,  from  the  standpoint  of  the  Yeasts,  a  mere 
incidental  factor  which  is,  so  to  speak,  unavoidable.  On  the 
other  hand,  from  the  broad  viewpoint,  the  waste  products 
of  the  action  of  the  Yeast  plants'  enzymes  represent  an  impor- 
tant phase  in  the  general  simplification  of  organic  compounds 
in  nature.  And  Man  turns  to  account  in  numerous  ways  both 
products  of  the  Yeasts'  destructive  powers  —  the  alcohol 
and  the  carbon  dioxide. 

Thus  the  Yeasts  are  practically  independent  of  free  oxygen 
and  in  this  they  agree  with  many  kinds  of  Bacteria  as  well  as 
some  animals,  chiefly  parasitic  worms,  which  are  able  to 
secure  the  necessary  oxygen  by  the  rearrangement  of  the 
atoms  within  a  molecule  or  the  disruption  of  the  molecule 


ADAPTATION    OF   ORGANISMS  311 

itself.  Indeed,  certain  species  of  Bacteria  not  only  do  not 
need  free  oxygen  at  all,  but  are  killed  when  it  is  present  in 
any  considerable  amount.  All  such  organisms  are  termed 
ANAEROBES.  A  common  example  is  Bacillus  tetani  which 
inhabits  garden  soil  and  street  dust  and  produces  tetanus, 
or  lockjaw,  in  Man  and  certain  domesticated  animals  when  it 
gains  entrance  to  the  tissues. 

TEMPERATURE.  Although  protoplasmic  activity  is  re- 
stricted to  ranges  of  temperature  which  do  not  seriously 
interfere  with  the  chemico-physical  processes  involved,  it  is 
a  commonplace  that  various  species  are  adapted  to  different 
degrees  of  temperature.  The  great  majority  of  organisms, 
however,  find  their  optimum  temperature  between  20°  C. 
and  40°  C.,  though  species  inhabiting  the  polar  and  tropical 
regions  show  adaptations  to  the  temperature  extremes  of 
their  surroundings.  As  a  matter  of  fact,  it  is  not  possible  to 
state  the  upper  and  lower  limits  beyond  which  active  life  is 
suspended,  but  some  Algae  and  Protozoa  are  known  to 
multiply  in  the  water  of  hot  springs,  certainly  at  temperatures 
higher  than  50°  C.,  and  others  in  water  until  freezing  actually 
occurs. 

But  many  of  the  lower  forms  of  life,  such  aos  the  Bacteria 
and  Protozoa,  have  the  power  of  developing,  particularly 
under  unfavorable  surroundings,  protective  coverings  of 
various  sorts  about  themselves  and  of  assuming  a  resting 
condition  in  which  all  the  metabolic  processes  characteristic 
of  active  life  are  reduced  to  the  lowest  ebb.  (Fig.  160.) 
In  this  spore  or  encysted  state  they  are  immune  to  extremes 
of  temperature  and  of  desiccation  to  which  they  readily 
succumb  during  vegetative  life.  Thus  some  types  of  Bacteria 
can  successfully  withstand  a  temperature  of  nearly  —200°  C. 
for  six  months,  and  about  — 250°  C.  for  shorter  periods,  which 
is  a  temperature  approaching  closely  that  at  which  no 


312 


FOUNDATIONS   OF   BIOLOGY 


chemical  reactions  are  known  to  occur.  Again,  the  spores 
of  other  Bacteria  can  endure  a  temperature  as  high  as  120°  C. 
for  a  short  time. 

It  is  clear  that  the  great  majority  of  organisms  are  at  the 
mercy  of  environmental  temperatures.  This  is  true  of  all 
except  the  higher  Vertebrates,  the  Birds  and  Mammals. 
These  so-called  warm-blooded,  or  HOMOTHERMAL,  animals 
possess  a  highly  complex  mechanism  which  maintains  their 


FIG.  160.  —  Spore  formation  and  germination  in  Bacteria.  A.  B,  C,  a 
pair  of  rods  forming  spores,  drawn  at  one  hour  intervals;  D,  a  five-celled 
rod,  with  three  fully-formed  spores,  which  was  allowed  to  dry  for  several 
days  and  then  placed  in  a  nutrient  medium;  E,  F,  the  same  spores  at 
one  and  three  hours  later;  G,  a  pair  of  typical  vegetative  rods.  (From 
Sedgwick  and  Wilson,  after  De  Bary.) 

body  temperature  practically  constant;  e.g.,  in  Man  at  37°  C. 
(98.6°  F.). 

The  heat  regulatory  mechanism  represents,  so  to  speak, 
the  culmination  of  the  assembling  and  elaborating,  during 
Vertebrate  evolution,  of  elements,  the  genesis  of  which  is 
found  among  the  Fishes.  In  the  Mammals  it  comprises 
insulating  material  in  the  skin,  a  closed  blood  vascular 
system,  power  of  rapid  oxidation,  endocrinal  and  other 
glandular  products,  evaporation  surface  of  the  lungs  and 
skin,  'trophic'  and  'temperature'  nerves,  coordinating 
centers,  etc.,  —  the  whole  complex  rendering  its  possessors 
largely  independent  of  the  surrounding  temperature  and 
making  possible  a  carrying  on  of  the  various  bodily  functions 
with  such  nicety  as  the  life  of  these  forms  demands. 


ADAPTATION    OF    ORGANISMS  313 

PRESSURE.  The  metabolism  of  organisms,  in  common 
with  chemical  processes  in  general,  is  influenced  by  the 
surrounding  mechanical  pressure.  Therefore  it  is  evident 
that  the  pressure  of  either  the  water  or  air  plays  an  important 
part  in  the  carrying  on  of  the  life  functions.  We  find  organ- 
isms adapted  to  the  greatest  depths  of  the  ocean  where  the 
water  pressure  is  several  hundred  atmospheres  —  so  great 
that  some  forms  burst  when  rapidly  brought  to  the  surface; 
while  others  are  adapted  to  live  at  high  altitudes  where  the  air 
pressure  is  relatively  low.  And  again,  the  higher  Vertebrates 
present  an  adaptive  mechanism  which  renders  them  less 
dependent  on  a  constant  barometric  pressure. 

These  few  examples  must  suffice  to  emphasize  the  general 
environmental  conditions  which  are  necessary  for  life,  as  we 
know  it,  to  exist,  and  to  suggest  that  within  these  broad 
limits  organisms  are  adapted  to  special  environmental  condi- 
tions so  that  there  is  scarcely  a  niche  in  nature  untenanted. 

2.   Adaptations  Essentially  Structural/ 

We  may  now  broaden  our  view  of  the  plasticity  of  organ- 
isms by  a  brief  consideration  of  adaptations  which  are 
essentially  structural .  But  here  as  elsewhere  it  is  absolutely 
impossible  to  divorce  structure  and  function  which,  ob- 
viously, are  only  reciprocal  aspects  of  the  fitness  of  living 
creatures. 

ADAPTIVE  RADIATION  OF  MAMMALS.  In  the  group  of 
Eutherian  Mammals,  forms  are  to  be  found  which  are  ex- 
traordinarily modified  in  adaptation  to  the  most  diverse 
environmental  conditions.  From  a  more  or  less  primitive 
type,  or  focus,  there  radiate,  as  it  were,  types  which  are 
specialized  for  different  habitats  and  modes  of  life.  (Fig. 
161.)  We  may  select  a  small  Malayan  insectivorous  animal 
known  as  Gymnura,  which  is  allied  to  the  Hedgehogs,  as 


314 


FOUNDATIONS   OF   BIOLOGY 


most  similar  among  living  Mammals  to  the  generalized  or 
focal  type  of  terrestrial  Mammal.  Gymnura  has  relatively 
short  pentadactyl  limbs  with  the  entire  palms  and  soles 


Cursorial- 
Unguligrade 


Volant 
(Aerial) 


Cursorial-Digi  tigrade 
(Terrestrial) 


Scansorial 
(Arboreal) 


Ambulatory 
(Terrestrial) 


Natatorial 
(Amphibious ) 


Short-limbed,  plantigrade, 
pentadactyl,  unguiculate 
stem 


Fossorial 
(Subterranean) 


(Aquatic) 

FIG.  161.  —  Diagram  of  the  adaptive  radiation  of  Eutherian  Mammals  as  exhibited 
in  limb  structure.     (From  Lull.) 

resting  flat  upon  the  ground  (PLANTIGRADE)  and  therefore 
essentially  adapted  for  comparatively  slow  progression. 
(Fig.  162.) 

Radiating  from  this  focus,  adaptations  for  rapid  running 
(cursorial  adaptations)  are  chiefly  evident  in  a  lengthening 


ADAPTATION   OF   ORGANISMS  315 

of  the  limbs.  Thus,  for  example,  in  the  Dogs,  Foxes,  and 
Wolves,  the  effective  limb  length  is  increased  by  raising 
the  wrist  and  heel  from  the  ground  and  walking  merely  upon 
the  digits  (DIGITIGRADE)  ; '  while  in  Antelopes,  Horses,  and 
hoofed  runners  in  general,  the  chief  limb  bones  themselves 
are  lengthened,  subsidiary  ones  are  suppressed,  and  the  wrist 
and  ankle  are  raised  still  further  from  the  ground,  so  that 
merely  the  tips  of  one  or  two  digits  of  each  limb  support 
the  animal  (UNGULIGRADE)  .  Thus  the  typical  cursorial 


FIG.  162.  —  Gymnura.     (From  Lull,  after  Horsfield  and  Vigors.) 

forms  represent  the  culmination  of  Mammalian  adaptation 
to  plains  and  steppes;  regions  in  which  long  distances  must 
frequently  be  traversed  in  quest  of  food,  and  safety  is  to  the 
swift.  (Fig.  163.) 

Another  line  of  adaptive  radiation  is  presented  by  the 
tree  dwellers :  arboreal  forms  which  make  their  own  the  / 
world  of  foliage  high  above  the  ground.  Such  are,  for 
instance,  the  Sloths  (Fig.  164),  which  are  really  tree  climbers 
that  walk  and  sleep  upside  down  suspended  from  branches; 
the  Man-like  Apes  that  swing  among  the  boughs  chiefly 
by  their  arms;  and  the  Squirrels  that  scamper  along  the 
branches.  Some  Squirrels  and  the  so-called  Flying  Lemurs 


316 


FOUNDATIONS    OF   BIOLOGY 


FIG.  163.  —  Foot  postures  of  Mammals. 
A,  plantigrade;  B,  digitigrade;  C,  unguli- 
grade.  (From  Lull,  after  Pander  and  D'Alton.) 


take  long  soaring  leaps 
supported  by  wide  folds 
of  skin  between  the  sides 
of  the  body  and  the  ex- 
tended limbs.  (Fig.  167.) 
But  the  Mammals  have 
not  left  the  air  untenanted, 
for  truly  volant  forms  are 
represented  by  the  Bats  in 
which  the  fore  limbs  with 
greatly  elongated  fingers 
form  the  framework  of 
true  wings.  (Fig.  168.) 

Passing  below  the  sur- 
face of  the  earth,  fossorial 
animals  are  found  such  as 
the  Woodchucks,  Gophers, 
and  especially  the  Moles, 
which  are  adapted  to  a 
subterranean  existence  by 
bodily  modifications  which 
facilitate  digging.  (Fig. 
165.)  The  gap  between  ter- 
restrial arid  aquatic  Mam- 
mals is  bridged  by  the 
Muskrats,  Beavers,  Otters, 
and  Seals  which  are  more 
or  less  equally  at  home  on 
land  and  in  the  water. 

The  truly  aquatic  Mam- 
mals are  the  Porpoises  and 
Whales  which  have  com- 
pletely abandoned  the 


ADAPTATION    OP   ORGANISMS 


317 


land  of  their  ancestors  of  the  geological  past  and  to-day 
approach,  in  adaptations  to  a  marine  life,  the  general  contour 
of  the  primitively 
adapted  aquatic  Ver- 
tebrates, the  Fishes. 
(Fig.  166.) 

Thus  the  various 
lines  of  adaptive  ra- 
diation of  the  Mam- 
mals from  a  general- 
ized terrestrial  type, 
such  as  Gymnura, 
have  provided  Mam- 
mals fitted  for  all  sorts  and  conditions  of  the  environment 
—  representatives  are  competing  with  members  of  other 


FIG.   164.  —  A  Sloth,   Choloepus,   walking  suspended 
from  a  branch.     (After  Allen.) 


FIG.  165.  —  Skeleton  of  a  Mole,  Talpa  europaea. 
Pander  and  D'Alton.) 


(After 


FIG.  166.  —  Skeleton  of  a  Porpoise.     The  vestigial  pelvic  bones  are  shown 
imbedded  in  the  flesh.     (After  Pander  and  D'Alton.) 

groups  beneath,  on,  and  above  the  earth  and  in  the  water. 
Somewhat  similar  adaptative  radiations  are  traceable  in  other 
animal  and  plant  groups,  though  there  seems  no  doubt  that 


318  FOUNDATIONS   OF   BIOLOGY 


FIG.  167.  —  'Flying  Lemur,'  Galeopithecus  volans.     (After  Lull.) 


FIG.  168.  —  A  Bat,  Vespertilio  noctula.     (After  Lull.) 


ADAPTATION    OF   ORGANISMS  319 

the  adaptability  of  the  Mammal  stock  —  its  potential  of 
evolution  —  is  in  no  small  degree  responsible  for  the 
dominant  position  which  the  Mammals  hold  in  the  animal 
world  of  to-day.  Man  is  a  Mammal. 

ANIMAL  COLORATION.  L  Perhaps  the  most  generally  strik- 
ing characteristic  of  organisms  is  their  color  and  color 
pattern.  Among  plants  this  applies  chiefly  to  the  flowers 
and  fruit  of  the  higher  forms,  though  here  and  there  through- 


FIQ.  169.  —  The  common  green  Katydid  (Microcentrum). 
(After  Riley.) 

out  the  plant  series  the  typical  green  color  is  replaced  or 
rendered  inconspicuous  by  others.  But  the  absence  of  photo- 
synthetic  pigments  in  animals  and  their  relatively  active  life 
have  permitted  more  latitude  in  body  color,  and  accordingly 
it  is  in  the  animal  world  that  color  adaptations  are  more 
numerous  and  varied.  Some  colors  and  color  patterns  are, 
of  course,  merely  incidental  to  the  chemical  composition  of 
the  whole  or  parts  of  the  body.  Others,  however,  irresistibly 
arouse  our  interest  and  seem  to  demand  a  less  simple  ex- 
planation because  they  are  apparently  of  special  service  to 
their  possessors.  A  few  examples  will  serve  to  bring  the 
problem  before  us  and  indicate  the  class  of  facts  involved. 
The  color  and  color  patterns  of  many  animals  are  such  that 
they  harmonize  or  fuse  with  the  usual  surroundings  of  the 
creatures  and  render  them  practically  indistinguishable  from 
their  immediate  environment.  Every  frequenter  of  the  open 


320 


FOUNDATIONS    OF    BIOLOGY 


knows  innumerable  instances.  The  song  of  the  green  Katy- 
did readily  guides  one  to  its  immediate  vicinity,  but  it  is  quite 
another  matter  to  distinguish  its  leaf-green  wings  among  the 
foliage  of  its  retreat.  (Fig.  169.)  One  is  attracted  by  the 


FIG.  170.  —  Catocala  lacrymosa;    A,  wings  expanded,  exposing  the  highly 
colored  hind-wings;   B,  resting  on  bark.     (From  Folsom.) 

striking  colors  of  an  Underwing  Moth  (Catocala)  while  in 
flight,  but  is  at  a  loss  to  find  the  insect  when  scarlet  or  orange 
is  obscured  by  the  overlapping  grayish-mottled  fore-wings 
blending  with  the  tree  trunk  where  it  has  come  to  rest.  (Fig. 


ADAPTATION    OF    ORGANISMS 


321 


The  white  of  the  Foxes,  Hares,  and  Owls  of  alpine  and 
arctic  regions;  the  green  color  of  foliage-dwelling  Insects 
and  Frogs;  the  tendency  toward  fawn  and  gray  of  desert 
Insects,  Reptiles,  Birds,  and  Mam- 
mals; the  olive  upper  surface  of 
the  bodies  of  brook  Fishes;  the 
steel  gray  above  and  white  below 
of  sea  Birds  which  harmonize  with 
sea  and  sky  when  viewed  from 
above  and  below  respectively  - 
the  number  of  such  cases  is  legion. 
Gazelles  living  on  the  lava  fields 
of  volcanic  regions  are  dark  gray, 
while  those  of  the  great  stretches 
of  sand  plains  are  white  —  the 
same  species  exhibiting  regional 
variations  in  color  which  blend 
with  the  surroundings.  Further- 
more, the  same  individual  may 
vary  in  color  with  the  seasonal 
changes  in  its  environment,  or 
present  different  color  schemes 
in  different  localities.  Thus  the 
summer  coat  colors  of  the  Arctic  Fox  and  the  Weasel  har- 
monize with  the  browns  of  rocks;  and  the  winter  coat  of 
white  with  snow-clad  nature.  And  the  Chameleons  are  by 
no  means  unique  in  their  ability  to  change  color  very  rapidly 
in  response  to  that  of  their  immediate  surroundings. 

But  confusion  is  worse  confounded  when  to  harmonizing 
color  is  added  harmonizing  form,  striking  examples  of  which 
are  the  Dead-leaf  Butterfly  (Kallima)  of  the  East  Indian 
region,  the  familiar  Walking-sticks  (Diapheromera),  and  the 
caterpillars  of  Geometrid  Moths.  (Figs.  171.  172,  173.) 


FIG.  171.  — Dead-leaf  Butterfly, 
Kallima  paralecta.  (After  Weis- 
mann.) 


322  FOUNDATIONS   OF   BIOLOGY 

Although  the  general  tendency  in  nature  is  for  sympathetic 
coloration  —  indeed,  it  is  frequently  possible  to  infer  from 


FIG.   172.  —  A  Walking-stick  Insect,  FIG.  173.  —  Larva  of  a  Geometric!  Moth 

Diapheromera    femorata,     on     a     twig.        resting  extended  from  a  twig.  (From  Jordan 
(From  Jordan  and  Kellogg.)  and  Kellogg.) 


the  color  of  an  animal  its  habitat  —  there  are  numerous  cases 
in  which  the  colors  and  color  schemes  seem  to  be  in  striking 


ADAPTATION  OF  ORGANISMS        323 

contrast  with  the  animal's  usual  background.  Sometimes, 
however,  the  contrast  which  is  so  striking  with  the  bird  in 
the  hand,  proves  to  be  'obliterative'  with  the  bird  in  the 
bush  —  a  conspicuous  color  pattern,  expressing  gradations 
of  light  and  shadow,  and  counter  shading,  fuses  with  a 
background  of  light  and  shadow  afforded  by  foliage. 

But  examples  of  color  patterns  which  by  the  most  liberal 
stretch  of  the  imagination  cannot  be  interpreted  as  harmoni- 


FIG.  174.  —  'Protective  Mimicry.'    A,  drone  Honey  Bee;  B,  a  Bee-fly, 
Eristalis  tenax.     (From  Folsom.) 


ous  with  the  animal's  usual  surroundings  are  not  far  to 
seek.  Brilliant  yellows  and  reds  render,  for  instance,  many 
Wasps,  Bees,  Butterflies,  and  various  species  of  Snakes  actu- 
ally conspicuous.  And  it  is  suggestive  that  very  many  of 
these  forms  are  provided  with  special  means  of  defense,  such 
as  poison  glands  and  formidable  jaws,  or  special  secretions 
which  render  them  unpalatable.  Moreover,  what  is  still 
more  interesting,  many  animals  possessing  this  'protective 
conspicuousness'  which  renders  them  easily  identified  and 
advertises  that  they  are  to  be  avoided  by  their  foes,  are 
frequently  'mimicked'  in  color  pattern  and  form  by  defenseless 
creatures.  Thus  commonly  associating  with  the  various 
species  of  Bees  hovering  about  flowers  are  defenceless  Flies 
which  are  so  bee-like  in  appearance  that  they  are  usually  mis- 


324  FOUNDATIONS    OF   BIOLOGY 

taken  for  Bees,  and  avoided  accordingly  by  human  and 
presumably  by  other  enemies  also.  (Fig.  174.) 

Now,  what  is  the  significance  of  such  phenomena  of  animal 
coloration  and  form  which  are  so  universal  in  nature?  The 
problem  appears  by  no  means  so  simple  to-day  as  it  did  a 
generation  ago,  and  biologists  are  not  so  ready  to  interpret 
individual  cases  as  'protective,'  'aggressive,'  'alluring,' 
'confusing,'  or  'mimetic.'  But  it  is  beyond  dispute  that  no- 
where else  is  the  plasticity  —  adaptability  —  of  organisms 
better  illustrated,  and  that,  taken  by  and  large,  such  adap- 
tations are  of  crucial  importance  in  the  life  and  strife  of 
species.  Whatever  may  be  the  origin  of  adaptive  variations, 
natural  selection  is  undoubtedly  responsible  for  their  ac- 
cumulation and  preservation. 

THE  LEGS  OF  THE  HONEY  BEE.  From  time  immemorial 
the  Honey  Bee  (Apis  mellifica)  has  been  the  subject  of  wonder 
and  study,  and  to-day  there  is  no  more  interesting  and  instruc- 
tive example  of  adaptation  than  that  exhibited  by  the  Bee  in 
relation  to  the  highly  specialized  community  life  of  the  hive. 

An  average  hive  comprises  some  65,000  Bees  of  which  one 
is  a  QUEEN,  several  hundred  are  DRONES,  and  the  rest  WORK- 
ERS. The  queen  is  the  only  fertile  female  and  accordingly 
she  is  the  mother  of  nearly  all  the  other  members  of  the  hive. 
Throughout  her  life  of  about  three  years  she  is  tended  and 
fed  by  her  numerous  offspring.  The  drones,  or  males,  con- 
tribute nothing  to  the  life  of  the  hive  in  which  they  live,  but 
at  the  swarming  of  the  Bees,  one  of  them  mates  with  a 
virgin  queen,  which  thenceforth  becomes  the  queen  of  a  new 
hive.  Thus  the  queen  and  the  drones  represent  an  adapta- 
tion of  the  colony  to  communal  life  —  a  physiological  di- 
vision of  labor  in  the  hive  which  involves  a  specialization 
of  a  class  solely  for  reproduction,  while  the  daily  work  and 
strife  of  the  colony  devolves  upon  the  workers.  The  latter 


ADAPTATION   OF   ORGANISMS  325 

are  sexually  undeveloped  females  which  do  not  lay  eggs  but 
spend  their  time  carrying  water,  collecting  nectar  and  pollen, 
secreting  wax,  building  the  comb,  preparing  food,  tending 
the  young,  and  cleaning,  airing,  and  defending  the  hive. 
(Fig.  175.) 

The  worker  is  a  'bundle  of  adaptations'  for  its  varied 
duties.  Indeed,  when  we  take  away  the  adaptations  there  is 
little  left!  The  primitive  insect  appendages  have  become 
specialized  in  the  worker  Bee  so  that  collectively  they  con- 
stitute a  battery  of  tools  adapted  with  great  nicety  to  the 


MALE  FEMALE  WORKER 

FIG.  175.  —  The  Honey  Bee,  Apis  mellifica.     (After  Shipley  and  MacBride.) 

uses  for  which  they  are  employed.  This  applies  to  all  of  the 
appendages  of  the  insect's  body,  but  we  shall  neglect  those  of 
the  head  (Fig.  176)  and  consider  only  the  specializations  of 
the  three  pairs  of  legs.  These,  as  in  all  Insects,  arise  from 
the  THORAX;  the  anterior  pair  from  the  first  segment  of  the 
thorax  (prothorax);  the  second,  or  middle,  pair  from  the 
second  thoracic  segment  (mesothorax) ;  and  the  posterior 
pair  from  the  third  and  last  thoracic  segment  (metathorax) . 
A  typical  insect  leg  consists  of  several  parts:  the  COXA, 
which  forms  the  junction  with  the  body,  followed  in  order  by 
the  TROCHANTER,  FEMUR,  TIBIA,  and  five-jointed  TARSUS,  or 
foot.  (Fig.  177.) 

The  worker  Bee's  PROTHORACIC  LEGS  show  the  following 
specializations.     The  femur  and  tibia  are  covered  with  long, 


326 


FOUNDATIONS   OF   BIOLOGY 


branched  FEATHERY  HAIRS  which  aid  in   gathering  pollen 
when  the  Bee  visits  flowers:  the  tibia,  near  its  junction  with 

the  tarsus,  bears  a  group 
of  stiff  bristles  (POLLEN 
BRUSH)  which  is  used  to 
brush  together  the  pol- 
len grains  that  have 
been  dislodged  by  the 

-  «<  hairs  °f  the  Upper  leg" 

?  segrnents>    Qn  the  oppo- 

site side  of  the  leg  is  a 
composite  structure,  the 

ANTENNA       CLEANER, 

formed  by  a  movable 
plate-like  process 
(VELUM)  of  the  tibia 
which  fits  over  a  circu- 
lar notch  in  the  upper 
end  of  the  tarsus.  The 
notch  is  provided  with 
a  series  of  bristles  which 
form  the  teeth  of  the 
antenna  COMB.  The 
antennae,  or  'feelers/ 
which  are  important  sense  organs  of  the  head,  are  cleaned 
by  being  placed  in  the  toothed  notch  and,  after  the  velum 
is  closed  down,  drawn  between  the  bristles  and  the  edge 
of  the  velum.  On  the  anterior  face  of  the  first  segment 
of  the  tarsus  is  a  series  of  bristles  (EYE  BRUSH)  which  is  used 
to  remove  pollen  and  other  particles  adhering  to  the  hairs  on 
the  head  about  the  large  compound  eyes  and  interfering  with 
their  operation. 

The  terminal  segment  of  the  tarsus  of  each  leg  is  provided 


Fio.  176.  —  Head  of  a  worker  Honey  Bee. 
a,  antenna;  6,  bouton;  g,  epipharynx;  I,  hypo- 
pharynx;  Ip,  labial  palpus;  m,  mandible;  mx, 
maxilla;  mxp,  maxillary  palpus.  (After  Cheshire.) 


ADAPTATION   OF   ORGANISMS 


327 


FIG.  177.  —  Legs  of  the  worker  Honey  Bee.  A,  outer  side  of  metathoracic  leg:  p, 
metatarsus  (first  segment  of  tarsus) ;  t,  tarsus;  ti,  tibia.  B,  inner  side  of  metathoracic 
leg:  c,  coxa;  p,  metatarsus;  t,  tarsus;  ti,  tibia;  tr,  trochanter;  wp,  pecten  and  auricle. 
C,  prothoracic  leg:  b,  pollen  brush;  eb,  eye  brush;  p,  metatarsus;  t,  tarsus;  ti,  tibia; 
v,  velum.  D,  mesothoracic  leg:  lettering  as  in  C;  s,  pollen  spur.  E,  joint  of  prothoracic 
leg:  lettering  as  in  C.  Fj  teeth  of  antenna  comb.  G,  transverse  section  of  tibia  through 
pollen  basket:  a,  antenna;  fa,  pollen;  h,  holding  hairs;  n,  nerve.  H,  antenna  in  process 
of  cleaning:  o,  antenna;  c,  antenna  comb;  I,  section  of  leg;  s,  scraping  edge  of  v,  velum. 
(From  Hegner,  after  Cheshire.) 


328  FOUNDATIONS   OF   BIOLOGY 

with  a  pair  of  notched  CLAWS,  a  sticky  pad  (PULVILLUS) 
and  TACTILE  HAIRS.  (Fig.  178.)  When  the  Bee  is  walking 
up  a  rough  surface,  the  points  of  the  claws  catch  and  the 
pulvillus  does  not  touch,  but  when  the  surface  is  smooth, 
so  that  the  claws  do  not  grip,  they  are  drawn  beneath  the 
foot.  This  change  of  position  applies  the  pulvillus,  and  it 
clings  to  the  smooth  surface.  Thus  the  character  of  the 
surface  automatically  determines  whether  claw  or  pul- 
villus shall  be  used.  But  there  is  another  adaptation  equally 


FIG.  178.  —  Foot  of  the  Honey  Bee  in  the  act  of  climbing,  showing  the  'automatic' 
action  of  the  pulvillus.  A,  position  of  foot  on  a  slippery  surface,  fh,  tactile  hairs;  pv, 
pulvillus;  t,  last  segment  of  tarsus;  a n,  claw.  B,  position  of  foot  in  climbing  on  a  rough 
surface,  an,  c,  claw.  C,  section  of  a  pulvillus  just  touching  a  flat  surface;  cr,  curved 
rod.  D,  the  same  applied  to  the  surface.  (From  Packard,  after  Cheshire.) 

remarkable.  "The  pulvillus  is  carried  folded  in  the  middle, 
but  opens  out  when  applied  to  a  surface;  for  it  has  at  its 
upper  part  an  elastic  and  curved  rod,  which  straightens  as 
the  pulvillus  is  pressed  down.  The  flattened-out  pulvillus 
thus  holds  strongly  while  pulled  along  the  surface  by  the 
weight  of  the  Bee,  but  comes  up  at  once  if  lifted  and  rolled 
off  from  its  opposite  sides,  just  as  we  should  pull  a  wet 
postage  stamp  from  an  envelope.  The  Bee,  then,  is  held 
securely  till  it  attempts  to  lift  the  leg,  when  it  is  freed 
at  once;  and,  by  this  exquisite  yet  simple  plan,  it  can  fix 
and  release  each  foot  at  least  twenty  times  per  second." 
(Cheshire.) 

The  characteristic  structures  of  the  middle  (MESOTHORACIC) 
legs  of  the  Bee  are  a  small  POLLEN  BRUSH  and  a  long  spine,  or 


ADAPTATION   OF   ORGANISMS  329 

SPUR,  which  is  employed  in  removing  the  pollen  from  the 
pollen  baskets  on  the  metathoracic  legs,  and  also  in  cleaning 
the  wings. 

The  METATHORACIC  LEGS  exhibit  four  remarkable  adapta- 
tions to  the  needs  of  the  insect  known  as  the  POLLEN  COMBS, 
PECTEN,  AURICLE,  and  POLLEN  BASKET.  The  pollen  combs 
comprise  a  series  of  rows  of  bristle-like  hairs  on  the  inner  sur- 
face of  the  first  segment  of  the  tarsus:  the  pecten  is  a  series 
of  spines  on  the  distal  end  of  the  tibia  which  is  opposed  by  a 
concavity,  the  auricle,  on  the  proximal  end  of  the  tarsal  seg- 
ment; while  the  pollen  basket  is  formed  by  a  depression  on 
the  outer  surface  of  the  tibia  which  is  arched  over  by  rows  of 
long  curved  bristles  arising  from  its  edges. 

Thus  the  worker  is  fully  equipped.  Flying  from  flower  to 
flower  for  nectar,  the  Bee  brushes  against  the  anthers  laden 
with  pollen,  some  of  which  adheres  to  the  hairs  on  its  body 
and  legs.  While  still  in  the  field,  the  pollen  combs  are  first 
brought  into  play  to  comb  the  pollen  from  the  hairs,  while 
the  pectens  scrape  the  pollen  from  the  combs.  Then  the 
auricles  are  manipulated  so  that  the  accumulating  mass  of 
pollen  is  pushed  up  into  the  bristle-covered  pollen  baskets. 
This  process  is  repeated  until  the  baskets  are  full  and  then 
the  insect  returns  to  the  hive,  where  the  contents  of  the  pollen 
baskets  are  removed  by  the  aid  of  the  spurs  with  which  the 
mesothoracic  legs  are  provided. 

Moreover,  the  structural  adaptations  of  the  worker  Bee 
are  but  one  aspect  of  a  reciprocal  fitness.  Many  of  the 
flowers  which  the  Bee  visits  show  remarkable  adaptations 
for  the  reception  of  the  Bee  and  for  dusting  it  with 
pollen,  because  Bees  are  effective  agents  in  transfer- 
ring pollen  from  flower  to  flower  and  thus  insuring  cross- 
fertilization. 


330  FOUNDATIONS   OF   BIOLOGY 

B.   ADAPTATIONS  TO  THE  LIVING  ENVIRONMENT 

We  have  now  discussed  the  close  reciprocal  relationship  be- 
tween organism  and  environment,  putting  emphasis  upon 
adaptations  to  the  non-living  surroundings,  and  must  turn 
more  specifically  to  some  striking  interrelations  of  organism 
with  organism,  in  order  to  make  possible  an  appreciation  of 
the  devious  means  to  which  they  have  recourse  —  to  what  ex- 
tent the  strands  of  the  web  of  life  become  entangled  —  in 
the  competition  for  a  livelihood. 

The  mutual  biological  interdependence  of  organisms  is.  in 
the  final  analysis,  the  result  of  the  primary  demands  of  all 
creatures  —  proper  food,  habitat,  reproduction,  defense 
against  enemies  and  the  forces  of  nature.  The  web  of  life 
is  an  expression  of  the  cooperation,  jostling,  and  strife  of 
individual  with  individual,  and  species  with  species  for  these 
primary  needs;  and  the  activities  which  follow  from  them 
form  the  foundations  of  life  in  the  lowest  as  well  as  the  high- 
est. There  is  a  struggle  for  existence.  A  common  food  Fish, 
the  Squeteague,  captures  the  Butter-fish  or  the  Squid,  which 
in  turn  have  fed  on  young  Fish,  which  in  their  turn  have  fed 
on  small  Crustacea,  which  themselves  have  utilized  micro- 
scopic Algae  and  Protozoa  as  food.  Thus  the  food  of  the 
Squeteague  is  actually  a  complex  of  all  these  factors,  and 
such  a  ' nutritional  chain'  is  no  stronger  than  its  single  links. 
Circumstances  which  modify  or  suppress  the  food  and  there- 
by reduce  the  abundance  of  the  microflora  and  microfauna  of 
the  sea  are  reflected  in  correlative  changes  in  the  abundance 
of  economically  important  food  Fishes.  And  this  same  prin- 
ciple is  true  throughout  living  nature,  though  only  occasion- 
ally is  it  possible  to  trace  it.  "Nature  is  a  vast  assemblage 
of  linkages." 


ADAPTATION    OF   ORGANISMS  331 

1.    Communal  Associations 

Perhaps  the  simplest  organismal  associations  are  repre- 
sented by  GREGARious~"animals,  such  as  Wolves  which  hunt 
in  packs,  and  Buffaloes  and  Horses  which  herd  for  protection. 
Here  the  association  is  more  or  less  temporary  and  there  is 
no  division  of  labor  between  the  members,  other  than  leader- 
ship by  one  animal. 

COMMUNAL  animals,  however,  exhibit  highly  complex  asso- 
ciations in  which  the  members  merge,  as  it  were,  their  indi- 
viduality in  that  of  the  community.  This  is  well  exhibited, 
for  example,  among  the  Ants,  in  which  all  of  the  various  spe- 
cies, about  5000  in  number,  are  communal,  and  in  the  Wasps 
and  Bees  in  which  all  gradations  exist  from  solitary  to  hive- 
dwelling  species.  And,  as  has  been  mentioned  in  the  case  of 
the  Bees,  the  division  of  labor  has  developed  to  the  extent 
that  structural  differentiations  have  given  rise  to  classes  of 
individuals  specially  adapted  for  the  performance  of  certain 
functions  in  the  economy  of  the  hive. 

It  is  in  Man,  however,  that  we  find  the  highest  expression 
of  communal  cooperation,  because  increased  intelligence,  in 
particular,  makes  flexible  the  stereotyped  life  as  exhibited  in 
the  lower  forms  —  the  human  individual  being  adaptable  to 
the  various  community  tasks. 

But  associations  are  not  confined  to  members  of  the  same 
species,  nor  are  all  an  expression  of  cooperative  adaptations. 
All  gradations  occur  from  those  which  are  mutually  beneficial 
to  the  parties  in  the  pact,  to  those  in  which  one  member 
secures  all  the  advantage  at  the  expense  of  the  other. 

2.   Symbiosis 

The  most  intimate  associations  in  which  the  organisms 
involved  are  mutually  benefited,  if  not  absolutely  necessary 


332  FOUNDATIONS   OF   BIOLOGY 

for  each  other's  existence,  are  termed  SYMBIOTIC.  A  familial- 
illustration  is  the  common  green  Hydra  (Hydra  viridis) 
which  owes  its  characteristic  color  to  the  presence  of  a  large 
number  of  unicellular  green  plants  which  live  in  its  endoderm 
cells.  The  products  of  the  photosynthetic  activity  of  the 
plant  cells  are  at  the  disposal  of  the  Hydra  and  the  latter 


FIG.  179.  —  The  formation  of  a  Lichen,  Pkyscia  paratina,  by  the  combination  of  an 
Alga  and  a  Fungus.  A,  germination  of  a  Fungus  spore  (sp),  whose  filaments  are  sur- 
rounding two  cells  (a)  of  the  unicellular  Alga,  Cystococcus  humicula.  B,  later  stage  in 
which  spores  have  formed  a  web  of  filaments  (mycelium),  enveloping  many  algal  cells. 
Magnified  about  400  times.  (From  Abbott,  after  Bonnier.) 

in  return  affords  a  favorable  abode  and  the  material  neces- 
sary for  the  life  of  the  plant. 

A  far  more  striking  example  of  symbiosis  is  afforded  by 
Lichens  which  represent  intimate  combinations  of  various 
species  of  Fungi  and  Algae.  (Fig.  179.)  In  each  case  the 
Fungus  supplies  attachment,  protection,  and  the  raw  mate- 
rials of  food,  while  the  green  Alga  performs  photosynthesis. 
Each  can  live  independently  under  favorable  conditions,  but 


ADAPTATION    OF   ORGANISMS 


333 


in  partnership,  they  are  superior  to  vicissitudes  with  which 
many  other  plants  cannot  cope,  and  thus  sonic  forms  become 
the  vanguard  of  vegetation  in  repopulating  rocky,  devas- 
tated areas. 

From  the  practical  standpoint  of  agriculture  the  symbiotic 
nitrogen-fixing  Bacteria  are  of  first  importance.    It  will  be 


FIG.  180.  —  Rose  Aphids  visited  by  Ants.     (After  Kellogg.) 

recalled  that  these  Bacteria  form  small  tubercles  on  the 
rootlets  of  higher  plants  and  make  atmospheric  nitrogen 
available  to  the  latter.  Thus  in  return  for  an  abode  and  cer- 
tain food  elements,  such  nitrogen-fixing  Bacteria  render  their 
symbiotic  associate  largely  independent  of  soil  nitrogen. 
Again  in  the  higher  animals,  including  Man,  evidence  is 
accumulating  which  indicates  that  certain  kinds  of  Bacteria 


334  FOUNDATIONS   OF   BIOLOGY 

find  their  normal  habitat  in  the  digestive  tract,  where,  inci- 
dental to  getting  their  own  living,  they  bring  about  chemical 
changes  in  the  food  of  their  host  which  is  an  important  factor 
in  the  digestive  processes  of  the  latter. 

Still  another  type  of  association  in  which  both  partners 
profit  is  represented  by  the  relation  that  occurs  between  Ants 
and  Aphids.  The  defenseless  Aphids  are  protected,  herded 
and  'milked'  by  the  Ants  to  supply  their  demand  for  honey- 
dew,  a  secretion  of  the  Aphids  which  the  Ants  greedily  de- 
vour. (Fig.  180.) 

3.   Parasitism 

But  associations  in  which  one  organism,  the  PAKASITE, 
secures  the  sole  advantage,  and  in  most  cases  at  the  expense 
of  the  helpless  second  party,  the  HOST,  are  far  more  numerous 
—  it  has  been  estimated  that  nearly  half  the  animal  kingdom 
are  parasites.  And  these  are  particularly  forced  upon  our 
attention  because  many  human  diseases  are  the  result  of 
Man's  unwilling  partnership  in  such  associations.  Indeed, 
PARASITOLOGY  has  become  an  important  subdivision  of  bi- 
ology, both  practical  and  theoretical.  Practical,  as  a  corner 
stone  of  public  health;  and  theoretical,  because  many  of  the 
most  remarkable  functional  and  structural  adaptations  are 

FIG.  181.  —  Diagram  illustrating  the  life  history  of  a  Malarial  Parasite.  The  stages 
above  the  line  of  dashes  occur  in  human  blood;  those  below,  in  the  body  of  a  Mosquito. 
I-V  and  6-10  show  asexual  multiplication  (schizogony)  in  human  red  blood  corpuscle 
following  introduction  of  a  parasite  (XIX)  by  a  Mosquito.  This  may  continue  by  the 
parasites  (10)  entering  other  corpuscles  until  a  large  number  of  the  latter  are  destroyed. 
Sooner  or  later  sexual  forms  arise.  VI-XIII,  the  sexual  generation  involving  the 
differentiation  of  male  (  $  )  and  female  (  2  )  gametes  which  unite  (XI)  to  form  a  zygote 
(XII).  The  zygote  becomes  motile  (XIII),  works  its  way  into  the  wall  of  the  stomach 
of  the  Mosquito,  and  encysts  (XIV).  Within  the  cyst  a  number  of  small  cells  (XVI, 
sp.bl.)  arise  by  division,  and  these,  in  turn,  give  rise  to  a  multitude  of  motile  cells 
(XVIII)  termed  sporozoites.  The  sporozoites  are  liberated  (XIX)  from  the  cyst, 
make  their  way  to  the  salivary  glands  of  the  Mosquito  where  they  are  ready  to  be 
inoculated  into  the  human  body,  and  so  gain  entrance  to  a  red  blood  corpuscle  (I). 
The  production  of  the  sporozoites  from  the  zygote  is  known  as  sporogony.  n,  nucleus 
of  the  narasite;  p,  pigment  and  waste  prorlunts  of  the  parasites;  fl,  long  slender  male 
gametes.  (From  Minchin,  in  Lankester's  Treatise.) 


336  FOUNDATIONS   OF   BIOLOGY 

exhibited  by  parasites  in  becoming  fitted  for  this  apparently 
highly  successful  method  of  gaining  a  livelihood,  and  by  the 
hosts  in  bearing  the  burden  with  the  least  outlay.  Generally 
speaking,  the  effect  on  the  parasite  consists  in  a  simplifica- 
tion of  the  various  organs  of  the  body  devoted  to  food- 
getting,  locomotion,  etc.,  since  their  duties  are  foisted  upon 
the  host;  while  the  organs  and  methods  of  reproduction  are 
highly  specialized  and  elaborated,  owing  to  the  necessity  of 
producing  enough  offspring  to  compensate  for  the  hazards 
involved  in  reaching  a  proper  host.  For  in  the  majority  of 
cases  a  parasite  is  adapted  to  live  in  a  specific  host,  and  death 
ensues  if  this  is  not  attained  at  the  proper  time. 

Probably  the  most  generally  interesting  example  of  para- 
sitism is  the  cause  of  the  disease  known  as  MALARIA.  Man  is 
subject  to  at  least  three  types  of  malaria,  each  the  result  of 
infection  by  a  different  malarial  organism.  The  malarial 
parasites  are  all  unicellular  animals,  Protozoa,  with  compli- 
cated life  histories  which  are  adaptations  to  the  specific  exi- 
gencies of  their  parasitic  existence.  (Fig.  181.)  One  part 
of  the  life  history,  the  asexual,  is  passed  in  the  red  blood 
corpuscles  of  Man;  while  the  other,  the  sexual,  occurs  in  the 
digestive  tract  of  certain  species  of  Mosquitoes.  A  single 
parasite  inoculated  into  the  human  system  by  the  bite  of  an 
infected  Mosquito  enters  a  red  blood  corpuscle  and  multi- 
plies. The  progeny,  liberated  from  the  destroyed  corpuscle, 
similarly  attack  other  corpuscles  and  multiply  until  a  very 
large  number  of  blood  corpuscles  are  destroyed.  And  the  liber- 
ation of  poisonous  products  of  the  life  processes  of  the  parasites 
provoke  the  chills  and  fevers  characteristic  of  the  disease. 

But  the  parasites  must  make  their  escape  before  the  human 
host  successfully  combats  the  toxic  substances,  kills  the 
parasites  by  taking  quinine,  or  succumbs  to  them.  The  get- 
away is  accomplished,  if  at  all,  by  a  Mosquito  biting  the  host 


ADAPTATION    OF   ORGANISMS  337 

and  taking  with  the  blood  certain  sexual  stages  of  the  para- 
site which  can  develop  in  the  cold-blooded  insect.  And  now 
the  Mosquito  is  the  host.  In  its  stomach  the  sexual  phase  of 
the  life  history  of  the  malarial  parasite  takes  place,  fertiliza- 
tion occurs,  and  finally  the  numerous  products  of  the  zygote 
work  their  way  to  the  mouth  parts  of  the  Mosquito,  where 
they  await  an  opportunity  to  enter  the  human  blood. 

The  life  history  of  malarial  parasites  exhibits  a  continuous 
series  of  adaptations  to  parasitic  life:   the  nicety  of  the  ad- 


FIG.  182.  —  A  trypanosome  (Trypanosoma  theileri)  from  the  blood  of  cattle.     Magni- 
fied about  3000  times.     (After  Liihe.) 

justment  being  especially  well  illustrated  at  the  transfer 
from  Man  to  Mosquito,  since  all  the  parasites  which  enter 
the  stomach  of  the  latter  are  digested  except  those  sexual 
forms  which  are  ready  to  initiate  the  sexual  part  of  the  cycle 
in  the  new  host. 

But  the  acme  of  parasitic  associations  is  only  attained 
when  the  adaptations  of  parasite  and  host  have  become  so 
complete  that  the  latter  'pays  the  price'  without  any  un- 
toward results.  Thus  the  Antelopes  and  similar  Mammals 
of  certain  regions  of  Africa  harbor  in  their  blood  various 
species  of  Protozoan  parasites,  known  as  TRYPANOSOMES, 


338  FOUNDATIONS   OF   BIOLOGY 

without  any  apparent  discomfort.  But  if  the  intermediate 
hosts,  which  are  biting  Flies,  transfer  for  example  Trypano- 
soma  brucei  to  imported  Horses  or  Cattle,  a  serious  disease 
results  which  is  usually  fatal.  Indeed,  the  opening  up  of  cer- 
tain regions  of  Africa  has  been  greatly  retarded  by  the 
ravages  of  this  Trypanosome  in  new  hosts  to  which  it  is  not 
adapted.  Generally  speaking,  pathogenic  species  may  be 
regarded  as  aberrant  forms  which  are  not  yet  adapted  to 
their  hosts  or  are  not  in  their  normal  hosts.  And  these  are 
the  parasites  which  are  forced  upon  our  attention,  though 
there  are  few  organisms  without  their  specially  adapted  para- 
sites —  the  parasites  themselves  not  excepted. 

4.   Immunity 

At  best,  however,  the  part  played  by  the  host  cannot  be 
regarded  as  ideal,  and  devious  types  of  adaptations  against 
parasites  exist  which,  insofar  as  they  are  effective,  bring 
about  IMMUNITY.  Usually  among  the  higher  animals,  includ- 
ing Man,  immunity  to  pathogenic  Bacteria  seems  to  have  its 
foundations  in  specific  chemical  substances  in  the  blood, 
termed  ANTIBODIES.  These  either  modify  the  activities  of 
certain  cells  of  the  body,  chiefly  the  white  blood  corpuscles, 
or  act  directly  upon  the  invaders  themselves  and  the  poisons 
(TOXINS)  which  they  produce.  The  white  blood  corpuscles 
have  been  called  the  'policemen  of  the  body7  because,  under 
the  influence  of  invading  organisms  and  of  certain  antibodies 
called  OPSONINS,  some  of  them  make  their  way  through  the 
walls  of  the  capillaries  in  the  region  of  the  infection  and,  in 
amoeboid  fashion,  engulf  and  digest  the  intruders.  When 
acting  in  this  capacity  the  corpuscles  are  referred  to  as 

PHAGOCYTES.  L 

Among  the  various  classes  of  antibodies  are  also  the  ANTI- 
TOXINS which  neutralize  the  poisonous  products  of  Bacteria, 


ADAPTATION   OF   ORGANISMS  339 

and  the  CYTOTOXINS  which  actually  destroy  the  foreign  cells. 
Various  specific  antibodies  may  be  naturally  present  in  the 
blood  —  a  part  of  the  heritage  —  so  that  an  individual  is 
immune  to  certain  diseases  due  to  pathogenic  organisms. 
Or  the  antibodies  may  be  produced  in  response  to  the  para- 
sites themselves,  and  the  individual  acquires  immunity  only 
after  undergoing  the  disease.  Finally,  immunity  may  be 
artificially  acquired  by  various  means,  such  as  VACCINATION, 
which  stimulate  the  production  of  antibodies  so  that  the 
individual  is  fore-armed,  as  it  were,  in  the  event  of  an  infec- 
tion. But  the  subject  of  immunity  has  become  a  science  in 
itself  within  the  past  few  years  —  a  science  which  has  as  its 
basis  the  exploitation  of  the  marvelous  power  of  adaptation 
of  protoplasm  as  exemplified  in  coping  with  disease-producing 
parasites. 

C.   INDIVIDUAL  ADAPTABILITY 

We  may  now  turn  to  a  survey  of  the  highest  expression 
of  adaptation  evolved  by  nature,  which  is  revealed  in  rela- 
tively simple  form  in  the  behavior  of  the  lower  organisms, 
gains  definiteness  and  content  as  we  ascend  the  animal  series, 
and  becomes  the  basis  of  the  intelligence  and  all  that  the 
mental  life  of  Man  involves.  It  is  the  adaptation  which 
renders  Man  essentially  superior  to  adaptation  —  enables 
him  to  a  large  extent  to  control,  instead  of  being  controlled 
by,  his  environment.  "It  seems  that  nature,  after  elaborat- 
ing mechanisms  to  meet  particular  vicissitudes,  has  lumped 
all  other  vicissitudes  into  one  and  made  a  means  of  meeting 
them  all"  -  the  nervous  mechanism. 

That  organisms  respond  to  environmental  changes,  we  are 
well  aware.  Life  itself  is  the  result  of  —  in  fact,  is  —  a  con- 
tinuous flow  of  physico-chemical  actions,  interactions,  and 
reactions  with  the  surroundings.  But  by  the  behavior  of 


340  FOUNDATIONS   OF   BIOLOGY 

the  organism  we  refer  specifically  to  the  reactions  of  the  or- 
ganism as  a  unit,  rather  than  to  the  internal  processes  in  the 
economy  of  its  life.  And  surveyed  from  a  broad  viewpoint, 
there  is  discernible  in  the  behavior  of  animals,  just  as  in  their 
structure  in  general  and  in  their  nervous  system  in  particu- 
lar, from  the  lowest  to  the  highest,  a  gradual  increase  in  the 
complexity  of  behavior.  The  behavior  of  Amoeba  or  Para- 
mecium  is  an  expression  of  the  primary  attributes  of  proto- 


FIG.  183.  —  Diagram  to  illustrate  the  avoiding  reaction  of  Paramecium.  A,  a 
solid  object  or  other  source  of  stimulation.  1-6,  successive  positions  taken  by  the 
animal.  The  rotation  on  its  long  axis  is  not  indicated.  See  Fig.  184.  (After  Jennings.) 

plasm — irritability,  conductivity,  and  contractility.  So  is  the 
behavior  of  Hydra  and  Earthworm,  in  which  special  cells 
constitute  a  definite  coordinating,  or  nervous,  system.  And 
so  is  the  complex  behavior  of  the  higher  animals,  including 
Man,  with  their  elaborate  series  of  sense  organs  and  highly 
developed  sensorium,  or  brain. 

"Let  us  now  try  to  form  a  picture  of  the  behavior  of  Par- 
amecium in  its  daily  life  under  natural  conditions.  An  indi- 
vidual is  swimming  freely  in  a  pool,  parallel  with  the  surface 
and  some  distance  below  it.  No  other  stimulus  acting,  it 
begins  to  respond  to  the  changes  in  distribution  of  its  internal 
contents  due  to  the  fact  that  it  is  not  in  line  with  gravity. 


ADAPTATION    OF   ORGANISMS 


341 


4 


It  tries  various  new  positions  until  its  anterior  end  is  directed 
upward,  and  continues  in  that  direc- 
tion. It  thus  reaches  the  surface  film. 
To  this  it  responds  by  the  avoiding 
reaction  (Fig.  183),  finding  a  new 
position  and  swimming  along  near 
the  surface  of  the  water.  .  .  .  Swim- 
ming forward  here,  it  approaches  a 
region  where  the  sun  has  been  shining 
strongly  into  the  pool,  heating  the 
water.  The  Paramecium  receives 
some  of  this  heated  water  in  the 
current  passing  from  the  anterior 
end  down  the  oral  groove.  (Fig. 
184.)  Thereupon  it  pauses,  swings 
its  anterior  end  about  in  a  circle, 
and  finding  that  the  water  coming 
from  one  of  the  directions  thus 
tried  is  not  heated,  it  proceeds  for- 
ward in  that  direction.  This  course 
leads  it  perhaps  into  the  region  of  a 
fresh  plant  stem  which  has  lately 
been  crushed  and  has  fallen  into  the 
water.  The  plant  juice,  oozing  out, 
alters  markedly  the  chemical  consti- 
tution of  the  water.  The  Parame- 
cium soon  receives  some  of  this 
altered  water  in  its  ciliary  current. 
Again  it  pauses,  or  if  the  chemical 

was  strong,  swims  backward  a  dis- 

t 

FIG.  184.  —  Diagram  to  show  the  rotation  on  the  long  axis,  and  the  spiral  path  of 
Paramecium.  1-4,  successive  positions  assumed.  The  dotted  areas  with  small  arrows 
represent  the  currents  of  water  drawn  from  in  front.  (After  Jennings.) 


342  FOUNDATIONS   OF   BIOLOGY 

tance.  Then  it  again  swings  the  anterior  end  around  in  a 
circle  till  it  finds  a  direction  from  which  it  receives  no 
more  of  this  chemical;  in  this  direction  it  swims  for- 
ward. .  .  . 

"In  this  way  the  daily  life  of  the  animal  continues.  It 
constantly  feels  its  way  about,  trying  in  a  systematic  way  all 
sorts  of  conditions,  and  retiring  from  those  that  are  harmful. 
Its  behavior  is  in  principle  much  like  that  of  a  blind  and 
deaf  person,  or  one  that  feels  his  way  about  in  the  dark. 
It  is  a  continual  process  of  proving  all  things  and  hold- 
ing to  that  which  is  good."  (Jennings.) 

The  behavior  of  Paramecium  leaves  one  with  the  impres- 
sion that  the  animal  is  largely  at  the  mercy  of  its  surround- 
ings —  that  the  environment  rather  than  the  organism  itself 
is  the  dominant  factor,  but  this  is  true  only  to  a  limited 
degree.  Paramecium  is  not  merely  an  automaton.'  Its  be- 
havior is  modifiable  and,  in  the  long  run,  is  adapted  to  the 
usual  changes  of  its  surroundings.  That  the  reactions  are 
adequate  for  the  simple  life  and  methods  of  reproduction  of 
Paramecium  is  attested  by  the  fact  that  it  is  one  of  the  most 
common  and  widely  distributed  animals. 

In  such  simple  beginnings,  then,  must  be  sought  the  largely 
automatic  responses  of  animals  to  the  exigencies  of  external 
conditions,  known  as  REFLEXES  and  INSTINCTS.  Both  are  the 
result  of  inherited  nervous  structure  and  therefore  may  be 
regarded  as  inherited  behavior  —  just  as  truly  characteristics 
of  the  organism  as  form  of  body  or  method  of  reproduction. 
And  increase  in  the  complexity  of  life  processes  has  involved 
a  synchronous  increase  in  the  number  and  complexity  of  in- 
stincts. The  primitive  reflexes  and  instincts  of  Hydra  lead 
it  to  seize  with  its  tentacles  small  organisms  within  reach 
and  pass  them  to  its  mouth:  the  Earthworm,  to  swallow 
decaying  leaves  as  it  burrows  through  the  soil:  the  Crayfish, 


ADAPTATION   OF   ORGANISMS  343 

to  grasp  its  prey  with  its  large  claws,  tear  it  into  pieces  by 
means  of  certain  appendages  about  the  mouth  which  are 
adapted  just  for  the  purpose  —  and  so  on  to  the  higher  Verte- 
brates where  the  feeding  instincts  reach  their  maximum  of 
complexity.  The  marvelous  behavior  of  Ants  and  Bees  is 
essentially  a  complex  of  instincts.  Turn  the  hive  around  and 
the  homing  instinct  of  the  Bees  proves  abortive  —  they  can- 
not find  the  entrance.  Moreover,  instincts  of  fear,  self- 
defense,  play,  care  of  the  young,  etc.,  render  a  considerable 
part  of  the  behavior  of  even  the  higher  organisms  more 
'automatic'  than  is  perhaps,  at  first  thought,  apparent.  (Fig. 
101.) 

But  just  as  the  behavior  of  Paramecium  and  its  allies  is 
modifiable,  so  instincts  which  seem  the  most  stereotyped 
show  at  least  a  slight  degree  of  adaptability  to  unusual  condi- 
tions. And  it  is  this  ever-present  modicum  of  modifiability, 
which  is  in  Man  called  /choice,'  that  leavens  the  whole  and 
becomes  the  dominant  factor  in  the  behavior  of  the  highest 
animals;  while  reflex  action  and  instinct  are  relegated  to  a 
subsidiary  though  by  no  means  unimportant  role. 

The  power  of  such  more  or  less  conscious  'choice'  of  re- 
sponses to  external  conditions  affords  a  gradual  and  ill- 
defined  transition  from  instincts  to  intellectual  processes,  or 
reason.  The  foundations  of  both  are  to  be  sought  in  simple 
reflex  actions  and  oft-repeated  voluntary  actions  which 
gradually  become  habits  —  relegated  to  the  level  of  reflex 
actions.  Indeed  a  large  part  of  the  education  of  Man  con- 
sists in  establishing  adaptive  reflexes  which  relieve  the 
conscious  life  of  innumerable  simple  factors  of  behavior,  and 
leave  it  more  or  less  free  for  the  higher  intellectual  processes. 
Although  it  is  necessary  to  emphasize  that  mind  and  intelli- 
gence, in  the  biological  sense,  are  expressions  for  that  inte- 
gration of  nervous  states  and  actions  which  makes  possible 


344  FOUNDATIONS   OF    BIOLOGY 

a  nicety  of  adaptation  of  behavior  to  environmental  condi- 
tions that  otherwise  would  be  impossible  —  that  it  is  our 
chief  means  of  adaptation  — •  "it  is  a  grave  mistake  to  mini- 
mize the  importance  of  the  great  gulf  between  Man's  nature 
and  that  of  the  most  highly  developed  of  the  lower  animals. 
In  no  respect  are  these  differences  more  marked  than  in  the 
various  forms  of  learning  that,  taken  together,  form  the 
.means  of  education."  (Cameron.) 

Thus  it  is  clear  that,  with  all  the  variations  in  structure  and 
function,  organisms  all  possess  irritability  in  common:  they 
all  exhibit  adaptive  responses  which  enable  them  to  exist  in 
spite  of  surrounding  changes.  "  Adaptability  appears  to  be 
the  touchstone  with  which  nature  has  tested  each  kind  of 
organism  evolved;  it  has  been  the  yard-stick  with  which  she 
has  measured  each  animal  type;  it  has  been  the  counter- 
weight against  which  she  had  balanced  each  of  her  produc- 
tions .  .  .  the  general  course  of  evolution  has  been  always 
in  the  direction  of  increasing  adaptability  or  increasing  per- 
fection of  irritability."  (Mathews.)  The  individual's  heri- 
tage affords  the  cumulative  result  of  the  adaptations  of  the 
race  —  including  adaptability. 


CHAPTER  XIX 
THE  ORIGIN  OF  SPECIES 

Thoughtful  men,  once  escaped  from  the  blinding  influences  of 
traditional  prejudice,  will  find  in  the  lowly  stock  whence  Man 
has  sprung,  the  best  evidence  of  the  splendor  of  his  capacities; 
and  will  discern  in  his  long  progress  through  the  Past,  a  reason- 
able ground  of  faith  in  his  attainment  of  a  nobler  Future. 

— Huxley. 

EVERYONE  recognizes  not  only  that  there  are  many  kinds 
of  animals  and  plants,  but  also  that  many  individuals  are 
essentially  the  same.  Groups  may  be  formed  of  individuals 
which  differ  less  among  themselves  in  the  sum  of  their  char- 
acters than  they  do  from  the  members  of  any  other  group 
of  individuals.  And  further,  the  members  of  a  group  produce 
other  individuals  which  are  essentially  similar.  Such  a 
group  of  similar  individuals  is  termed  by  the  biologist  a 
SPECIES.  It  will  be  noted,  therefore,  that  a  species  is  merely 
a  concept  of  the  human  mind  —  the  only  reality  in  nature 
is  the  individual,  and  an  understanding  of  the  differences  be- 
tween individuals  gives  us  the  key  to  the  differences  between 
species.  This  seemingly  obvious  point  of  view  has  but 
relatively  recently  been  clearly  emphasized  by  biologists,  and 
the  species  rather  than  the  individual  has  loomed  large 
in  the  discussions  of  how  plants  and  animals  came  to  be  what 
they  are  to-day. 

From  the  time  of  the  Greek  natural  philosophers  there 
always  have  been  men  who  have  sought  a  naturalistic  expla- 
nation of  the  origin  of  the  diverse  forms  of  animals  and  plants, 

345 


346  FOUNDATIONS   OF   BIOLOGY 

and  who  have  suggested  that  the  present  ones  arose  from  ear- 
lier forms  by  a  process  of  descent  with  modification,  or  EVO- 
LUTION. But  with  the  revival  of  natural  history  studies  after 
the  Middle  Ages,  the  Mosaic  account  of  creation  led  the 
majority,  perhaps  almost  unconsciously,  to  assume  that  there 
are  as  many  kinds  of  organisms  as  issued  from  the  Ark.  And 
this  is  not  so  strange,  as  might  at  first  glance  appear,  when 
one  considers  that  all  of  the  important  facts  which  we  have 
reviewed  in  the  preceding  pages  were  then  absolutely  un- 
known, and  that  the  number  of  known  kinds  of  animals 
totalled  but  a  thousand  or  so,  instead  of  upward  of  a  million, 
as  to-day. 

The  pioneer  work  of  the  early  Renaissance  naturalists 
consisted  principally  of  collecting  and  describing  animals 
and  plants.  This  involved  making  a  catalog  of  the  different 
kinds  —  classifying  them  in  some  way  —  and  consequently 
some  basis  of  classification  was  sought.  Thus  attention  was 
focused  on  the  kinds  of  species  and  for  practical,  if  for  no 
other,  reasons,  the  species  assumed  a  prominence  which  over- 
shadowed the  individuals  which  composed  it.  As  a  matter  of 
fact  during  the  eighteenth  century  the  greatest  student  of 
plant  and  animal  classification,  Linnaeus,  emphasized  the 
idea  that  each  species  represents  a  distinct  thought  of  the 
Creator  and  that  the  object  of  classification  is  to  arrange 
species  in  the  order  of  the  Creator's  consecutive  thoughts. 
This  viewpoint  is  somewhat  whimsically  expressed  by  the 
old  naturalist  who,  finding  a  beetle  which  did  not  seem  to 
agree  exactly  with  any  species  in  his  collection,  solved  the 
difficulty  by  crushing  the  unorthodox  individual  under  his 
foot.  (See  page  391.) 

We  may  consider  that  the  consensus  of  opinion  up  to  the 
middle  of  the  last  century  was  overwhelmingly  on  the  side 
of  SPECIAL  CREATION  and  FIXITY  OF  SPECIES,  and  there- 


THE    ORIGIN   OF   SPECIES  347 

fore  against  the  idea  occasionally  advanced  by  men,  as  it  now 
appears,  ahead  of  their  times,  that  DESCENT  WITH  MODIFICA- 
TION is  the  explanation  of  the  origin  of  the  diverse  forms 
of  plants  and  animals.  But,  as  nearly  every  one  knows,  a 
complete  reversal  of  opinion  has  occurred  since  1860  —  to- 
day professional  scientists  and  most  educated  laymen 
accept  ORGANIC  EVOLUTION.  And  we  have  accepted  it  in  the 
preceding  sections  of  this  work;  but  if  this  appears  to  have 
been  prejudging  the  question,  the  explanation  is  that  the 
genetic  connection  of  organisms  is  the  guiding  principle  of  all 
biology  —  and  the  mere  fact  that  an  unbiased  presentation 
of  the  data  seems  to  prejudge  the  question  is  the  most  cogent 
presumptive  evidence  for  evolution.  It  is  true  that  there  are 
wide  differences  of  opinion  among  biologists  in  regard  to  the 
factors  which  have  brought  about  the  evolutionary  change  — 
but  there  are  none  in  regard  to  the  fact  of  evolution  itself.  It 
will  be  convenient,  therefore,  first  to  summarize  the  evidences 
of  evolution  and  then  to  discuss  modern  views  in  regard  to 
the  methods  of  evolution.  (See  Glossary,  'evolution.') 

A.  EVIDENCES  OF  ORGANIC  EVOLUTION 

To  one  who  has  thoughtfully  followed  the  preceding  pages 
there  must  immediately  occur  many  facts  which  are  readily 
and  reasonably  interpreted  from  the  point  of  view  of  descent 
of  one  species  from  another,  but  which  are  entirely  enig- 
matical from  that  of  the  special  creation  of  species.  For 
instance,  one  will  recall  the  cellular  structure  of  all  organisms; 
the  method  of  origin  and  the  fate  of  the  germ  layers  in  ani- 
mals; the  interrelationship  of  the  urinary  and  reproductive 
systems  in  the  Vertebrates;  the  comparative  anatomy  of 
the  vascular  and  skeletal  systems  of  Vertebrates;  the  simi- 
larity of  the  physical  basis  of  inheritance  in  animals  and 
plants;  the  gradual  dominance  of  the  sporophyte  over  the 


348  FOUNDATIONS   OF   BIOLOGY 

gametophyte  from  the  lower  to  the  highest  plants;  and 
so  on. 

In  general,  such  is  the  nature  of  the  data  which  support 
the  evolution  theory.  Although  the  evidence,  from  the 
nature  of  the  case,  must  be  indirect,  it  is  none  the  less  cogent 
chiefly  because  the  facts  for  evolution  are  from  such  diverse 
sources  and  all  converge  toward  the  same  conclusion.  The 
theory  of  evolution  reaches  the  highest  degree  of  probability, 
since  in  every  branch  of  botany  and  zoology  all  the  data 
are  most  simply  and  reasonably  explained  on  the  basis  of 
'descent  with  modification/  and  not  a  single  fact  points  to- 
ward special  creation.  It  is  a  cardinal  principle  of  science 
to  accept  the  simplest  conceptions  which  will  embrace  all 
the  facts. 

We  may  now  summarize  some  of  the  most  striking  evi- 
dence from  taxonomy,  comparative  anatomy,  paleontol- 
ogy, embryology,  physiology,  and  distribution  of  ani- 
mals. But,  as  will  soon  appear,  it  is  impossible  to  arrange 
the  facts  in  hard  and  fast  groups  under  these  headings 
-the  evidence  from  one  merges  into  that  from  another, 
and  in  the  final  analysis  nearly  all  are  based  on  compara- 
tive anatomy  in  the  broadest  sense  of  the  term. 

1.    Taxonomy 

When  the  serious  study  of  classification  was  well  under 
way,  biologists  found  increasing  evidence  of  the  similarity, 
or  affinity,  of  various  SPECIES  of  animals  and  plants.  Not 
only  is  it  possible  to  arrange  animals,  for  example,  in  an 
ascending  series  of  increasingly  complex  forms,  but  also  in 
many  cases  it  is  difficult  or  impossible  to  decide  just  where 
one  species  ends  and  the  next  begins.  That  is,  the  most  aber- 
rant individuals  within  a  given  species  frequently  approach 
those  of  a  closely  similar  species.  There  are  intergrades. 


THE    ORIGIN   OF   SPECIES  349 

Again,  it  is  found  that  species  themselves  can  be  naturally  ar- 
ranged in  more  comprehensive  groups  to  which  the  name 
GENUS  is  applied.  For  example,  the  common  Gray  Squirrel 
represents  the  species  carolinensis,  and  the  Red  Squirrel,  the 
species  hudsonicus.  Both  are  obviously  Squirrels,  and  there- 
fore both  species  are  grouped  under  the  genus  Sciurus.  Ac- 
cordingly, each  animal  is  given  a  name  composed  of  two 
words:  the  first,  generic  and  the  second,  specific.  The  Gray 
Squirrel  is  Sciurus  carolinensis  and  the  Red  Squirrel  is  Sciurus 
hudsonicus.  Thus  to  give  a  scientific  name  to  an  animal  or 
plant  is  really  to  classify  it,  because  the  first  word  of  the 
name  indicates  that  it  possesses  some  fundamental  char- 
acteristics in  common  with  the  other  species  of  the  genus  — 
in  fact,  is  more  like  them  than  it  is  like  any  other  group  of 
organisms. 

But  again,  the  members  of  the  genus  Sciurus  have  many 
characteristics  in  common  with  other  animals  which  obvi- 
ously are  not  true  squirrels.  The  Chipmunks  or  Ground 
Squirrels,  for  instance,  differ  not  only  in  certain  obvious 
features,  but  in  the  possession  of  internal  cheek  pouches, 
etc.  This  dissimilarity  and  similarity  is  expressed  by  placing 
them  in  a  different  genus,  Tamias,  but  in  the  same  FAMILY, 
Sciuridae.  The  familiar  eastern  Chipmunk  is  Tamias 
striatus. 

Moreover,  while  the  Beaver  (Castor  americana)  differs 
still  more  from  the  Squirrels  than  do  the  Chipmunks,  and 
therefore  is  placed  in  a  distinct  family,  the  Castoridae,  it 
nevertheless  agrees  with  both  in  many  fundamental  ways,  so 
that  it  is  placed  in  the  ORDER  Rodentia,  which  also  includes 
the  Squirrels  and  Chipmunks,  as  well  as  many  other  families 
and  genera.  Other  orders,  such  as  the  Ungulata  (Horses, 
Cattle,  etc.)  and  the  Carnivora  (Cats,  Dogs,  Bears,  etc.), 
while  they  differ  widely  from  the  Rodents,  still  agree  with 


350  FOUNDATIONS   OF   BIOLOGY 

them  in  possessing  hair,  and  milk  glands  for  suckling  the 
young.  This  basic  likeness  is  expressed  by  including  all  un- 
der the  CLASS  Mammalia. 

The  Mammals  in  turn  are  readily  distinguished  from  Birds, 
Reptiles,  Amphibians,  and  Fishes  (each  of  which  forms  a  sepa- 
rate class),  but  nevertheless  are  constructed  on  the  same 
fundamental  plan,  comprising  a  dorsal  central  nervous  sys- 
tem surrounded  by  skeletal  elements  forming  the  skull  and 
spinal  column.  Therefore,  all  are  comprehended  in  the  larger 
group  Vertebrata,  in  contrast  with  the  Invertebrate  groups 
which  include  Hydra,  Earthworm,  Crayfish,  etc.  (See  pp.  116, 
146,  414.)  The  classification  of  the  Gray  Squirrel,  Stiurus 
carolinensis,  (Fig.  86.)  may  be  outlined  as  follows: 

SUBPHYLUM  —  Vertebrata. 
CLASS  —  Mammalia. 
ORDER  —  Rodentia. 
FAMILY  —  Sciuridae. 
GENUS  —  Sciurus. 

SPECIES  —  S.  carolinensis. 

This  classification  of  the  Gray  Squirrel,  although  it 
incidentally  serves  to  illustrate  the  general  method  of  classi- 
fication of  all  organisms,  is  important  because  it  places  con- 
cretely before  us  the  fact  that  organisms  show  such  funda- 
mental similarities  with  obvious  dissimilarities.  In  short, 
the  mere  fact  that  animals  and  plants  naturally  arrange 
themselves,  as  it  were,  in  classes,  orders,  families,  genera, 
species,  etc.,  raises  the  question  of  the  origin  of  species.  Is 
special  creation  implying  fixity  of  species,  or  is  descent  with 
modification  the  more  plausible  explanation? 

The  unavoidable  answer  is,  descent  with  modification  — 
evolution  —  because  the  principle  in  accordance  with  which 
the  groups  of  increasing  comprehensiveness  are  formed  is 


THE   ORIGIN   OF   SPECIES  351 

solely  the  greater  or  less  similarity  in  the  structural  features 
of  the  organisms.  It  its  much  more  reasonable  to  assume 
that  the  thread  of  fundamental  similarity  which  runs  through 
all  the  Vertebrates,  for  instance,  is  the  result  of  inheritance, 
while  the  differences  of  orders,  families,  genera,  etc.,  are 
due  to  changes  brought  about  under  different  unknown 
conditions,  than  it  is  to  assume  that  each  is  the  result  of 
a  special  creative  act.  Especially  so  when  we  realize  that  in 
a  very  large  number  of  cases  it  is  difficult  or  impossible  to 
decide  the  limits  of  a  species,  owing  to  variations  among 
the  individuals  comprising  it,  and  it  is  necessary  to  resort 
to  subspecies  and  varieties  in  classification.  And  further, 
among  genera,  intergrading  forms  demand  subgenera;  among 
orders,  suborders;  among  classes,  sub-classes;  and  so  on.  If 
we  admit  the  origin  by  descent  with  modification  of  the  sub- 
species and  varieties,  there  is  no  logical  reason  for  denying 
the  same  origin  of  species,  orders,  and  higher  groups.  The 
difference  is  one  of  degree  and  not  of  kind.  Before  the  recog- 
nition of  evolution  classification  was  a  groping  after  an 
elusive  ideal  arrangement  which  naturalists  felt  but  were 
unable  to  express  except  in  artificial  form  and  in  transcenden- 
tal terms.  Under  the  influence  of  the  evolution  theory  classi- 
fication became  the  natural  expression  of  biological  pedigrees. 

2.   Comparative  Anatomy 

The  evidence  from  taxonomy  is,  as  has  just  been  seen,  really 
evidence  from  comparative  anatomy,  since  modern  classifi- 
cations are  based  chiefly  on  anatomical  characters.  The 
various  groups  —  classes,  orders,  families,  genera,  species, 
etc.  —  are  founded  not  on  a  single  difference,  nor  on  several 
differences,  but  on  a  large  number  of  similarities.  For  in- 
stance, the  differences  exhibited  throughout  the  five  classes 
of  the  Vertebrates  are  relatively  slight  in  comparison  with 


352 


FOUNDATIONS   OF   BIOLOGY 


THE    ORIGIN    OF    SPECIES  353 

the  fundamental  resemblances.  This  similarity  in  dis- 
similarity is  brought  out  by  the  science  of  comparative 
anatomy.  A  few  concrete  examples,  some  of  which  we  are 
already  familiar  with,  will  serve  to  bring  the  main  facts  clearly 
before  us. 

The  fore-legs  of  Frogs  and  Lizards,  the  wings  of  Birds,  the 
fore-legs  of  the  Horse,  and  the  arms  of  Man  are  built  on  the 
same  basic  plan.  (Figs.  80,  81,  185,  186.)  The  same  is  true  of 
the  hind-limbs.  Clearly  all  are  homologous  structures,  such 
variations  as  exist  being  brought  about  chiefly  by  the  modi- 
fication or  absence  of  one  part  or  another.  In  short,  all  the 
chief  parts  of  both  the  fore-limbs  and  the  hind-limbs  are 
homologous  throughout  the  series.  All  are  composed  of 
the  same  fundamental  materials  disposed  in  practically 
the  same  way  —  nearly  all  the  bones,  muscles,  blood  vessels, 
and  nerves  are  homologous.  Or  compare  the  digestive  sys- 
tems of  the  same  forms,  or  the  excretory  and  reproductive 
systems.  One  has  but  to  recall  that,  on  an  earlier  page,  it 
was  possible  to  describe  in  general  terms  these  systems  as 
they  exist  throughout  the  Vertebrate  series  —  in  forms  as 
obviously  different  as  Fish  and  Man.  They  are  all  funda- 
mentally the  same.  (Figs.  82-87,  97.) 

Turning  to  the  Invertebrates,  we  may  remind  the  reader 
that  all  the  appendages  of  the  Crayfish  are  built  on  the  same 
simple  biramous  plan  as  exhibited  in  the  swimming  legs 
(swimmerets)  of  the  abdomen.  The  highly  specialized 
walking  legs,  great  claws,  jaws,  and  feelers  (antennae  and 
antennules)  are  all  reducible  to  modifications  of  the  simple 
swimmeret  type.  (Fig.  72.)  In  short,  all  are  homologous 
structures,  though  differing  widely  in  function.  This  is  a 
most  striking  example  of  SERIAL  HOMOLOGY,  though  we  have 
seen  the  same  principle  exhibited  in  the  Vertebrates  where 
the  fore-limbs  and  the  hind  -limbs  of  each  animal  are  homolo- 


354 


FOUNDATIONS   OF   BIOLOGY 


Fio.  186.  —  Skeletons  of  Man  and  of  Gorilla.     (From  Lull.) 


THE    ORIGIN    OF    SPECIES 


355 


gous.  Moreover,  the  appendages  of  the  Crayfish  are  not  only 
serially  homologous  among  themselves,  but  are  also  homolo- 
gous with  the  appendages  of  all  the  other  members  of  the 
class  Crustacea  —  just  as  the  limbs  of  one  Vertebrate  are 
homologous  with  those  of  all  other  Vertebrates. 

Another  class  of  facts  presented  by  comparative  anatomy 
is  derived  from  the  so-called  VESTIGIAL  organs.  In  Man 
there  are  nearly  a  hundred  structures  which  at  best  are 
useless  and  sometimes  are  harmful.  One  thinks  at  once  of 
the  VERMIFORM  APPENDIX  of  the  large  intestine,  a  remnant 
of  an  organ  which  serves  a  useful  purpose  in  the  vegetable- 
feeding  (herbivorous)  Mammals.  (Fig.  88.)  But  equally 
suggestive  are  the  muscles  of  the  ear,  which  in  some  indi- 
viduals are  sufficiently  developed  to  move  the  external  ear; 
the  so-called  third  eyelid  at  the  inner 
angle  of  the  eye  which  corresponds 
to  the  lid  (NICTITATING  MEMBRANE) 
that  moves  laterally  across  the  eye 
in  Bird  and  Frog;  or  the  terminal 
vertebrae  (COCCYX)  of  the  human 
spinal  column  which  are  remnants 
of  the  tail  of  lower  Vertebrates. 
(Fig.  87.) 

Other  animals  are  likewise  replete 
with  such  structures.  Porpoises 
possess  vestiges  of  hind-limbs  en- 
closed within  the  body,  and  cer- 
tain species  of  Snakes  have  tiny  use- 
less hind-legs.  The  'splint  bones'  of 
the  Horse  are  remnants  of  lost  toes. 

Among  plants,  it  will  suffice  to  mention  the  functionless 
remnant  of  the  pistil  which  sometimes  is  present  in  'male' 
(staminate)  flowers.  (Figs.  166,  187,  189.) 


FIG.  187.  —  Vestigial  hind- 
limbs  of  a  Snake,  Python,  f, 
femur  or  thigh  bone;  il,  ilium 
or  hip  bone.  (From  Romanes.) 


356  FOUNDATIONS   OF   BIOLOGY 

In  another  class  of  cases,  the  organs,  or  remnants  of  organs, 
of  a  lower  form  are  altered  or  completely  made  over,  as  it 
were,  into  new  organs  of  the  higher  form.  During  the  embry- 
onic life  of  all  Vertebrates  there  are  gill  slits,  all  of  which  soon 
vanish  except  one,  which  remains  as  an  opening  (EUSTACHIAN 
TUBE)  connecting  the  middle  ear  with  the  pharynx.  (Fig. 
110.) 

Gill  arches,  which  function  as  supports  for  the  gills  in  the 
aquatic  Vertebrates,  persist  in  highly  modified  form  as  skele- 
tal structures  associated  with  the  tongue  and  entrance  to  the 
lungs  (LARYNX)  in  terrestrial  forms.  The  milk  glands  of 
Mammals  are  transformed  sebaceous  glands  of  the  skin, 
while  the  poison  glands  of  Snakes  are  specialized  salivary 
glands  of  the  mouth.  Finally,  in  this  connection  the  reader 
will  recall  the  transformations  of  the  blood  vessels  in  the 
Vertebrates  which  occur  with  the  substitution  of  lungs  for 
gills,  and  also  the  variations  and  interrelationships  of  the 
excretory  and  reproductive  systems  in  the  ascending  series 
of  Vertebrate  classes.  (Figs.  95,  97.) 

One  may,  of  course,  conclude  from  all  these  facts  that  Fish, 
Frog,  Lizard,  Bird,  and  Man  have  each  been  independently 
created  according  to  the  same  preconceived  plan  —  and  like- 
wise all  the  great  numbers  of  orders,  families,  genera,  species, 
etc.,  of  each  of  the  five  classes  that  these  forms  represent. 
Or,  one  may  conclude,  that  all  have  arisen  by  descent  with 
modification  from  a  primitive  Vertebrate  organism  which 
possessed  the  fundamental  similarities  exhibited  from  Fish 
to  Man.  The  latter  is  the  conclusion  accepted  unreservedly 
by  biologists  to-day. 

3.   Paleontology 

Huxley  once  said  that  if  zoologists  and  embryologists  had 
not  put  forward  the  theory  of  evolution,  it  would  have  been 


THE   ORIGIN   OF   SPECIES  357 

necessary  for  paleontologists  to  invent  it.  What  then  are  the 
main  facts  offered  by  the  study  of  the  fossil  remains  of  extinct 
animals  and  plants? 

In  the  first  place  it  must  be  made  clear  that  geologists  are 
able  to  determine,  with  remarkable  accuracy  in  most  cases, 
the  sequence  in  time,  or  CHRONOLOGICAL  SUCCESSION,  of  the 
rock  strata  composing  the  Earth's  surface.  The  main  outline 
of  this  scheme  of  geological  chronology  was  understood  long 
before  the  evolution  of  organisms  was  a  crucial  question;  so 
that  we  may  consider  the  evidence  which  it  affords  of  the 
chronological  succession  of  the  fossil  remains  exhibited  by 
the  various  strata,  as  impartial  testimony  to  the  order  of  ap- 
pearance on  the  Earth  of  the  different  types  of  animals  and 
plants. 

The  following  geological  time-table  summarizes  the 
panoramic  succession  of  life  as  it  is  seen  by  the  paleontolo- 
gist. It  is  useless  to  attempt  to  state  the  absolute  duration 
of  geologic  time,  because  we  have  little  more  than  guesses  to 
depend  on,  though  there  are  fairly  reliable  data  in  regard  to 
the  relative  length  of  the  various  eras.  Perhaps  the  conserva- 
tive estimate  of  500,000,000  years  —  at  least  half  of  which 
was  before  the  Permian  period  —  will  serve  to  spell  the 
Earth's  unfathomable  past. 


THE  GEOLOGICAL  TIME-TABLE1 

PRESENT  TIME. 
PSYCHOZOIC  ERA.    AGE  OF  MAN  OR  AGE  OF  REASON. 

Includes  the  present  or  'Recent  time/  and  the  time  during 
which  Man  attained  his  highest  civilization,  estimated  to  be 
probably  less  than  30,000  years. 

GEOLOGIC  TIME. 

CENOZOIC  ERA.    AGE  OF  MAMMAL  DOMINANCE. 
Glacial  or  Pleistocene  time.    Last  great  ice  age. 
Late  Cenozoic  or  Pliocene  and  Miocene  time.    Primates  changing 

into  Apes  and  Man. 
Early  Cenozoic  or  Oligocene  and  Eocene  time.    Rise  of  higher 

Mammals,  including  Primates. 
MESOZOIC  ERA.    AGE  OF  REPTILE  DOMINANCE. 
Cretaceous  period.    Rise  of  primitive  Mammals. 
Comanchian  period.    Rise  of  Flowering  Plants  and  higher  In- 
sects. 

Jurassic  period.    Rise  of  Birds  and  flying  Reptiles. 
Triassic  period.    Rise  of  Dinosaurs,  and  Mammalian  stock. 
PALEOZOIC  ERA.    AGE  OF  FISH  DOMINANCE. 

Permian  period.    Rise  of  Reptiles.    Another  great  ice  age. 
Pennsylvanian  period.    Rise  of  Insects  and  first  time  of  marked 

coal  accumulation. 

Mississippian  period.    Rise  of  marine  Sharks. 
Devonian  period.    First  known  marine  Fishes,  and  Amphibians. 
Silurian  period.     First  known  land  floras. 
Ordovician  period.    First  known  fresh-water  Fishes. 
Cambrian  period.     First  abundance    of    marine    fossils,    and 

dominance  of  Trilobites. 
PROTEROZOIC  ERA.     AGE   OF  INVERTEBRATE   DOMINANCE. 

An  early  and  a  late  ice  age. 
ARCHEOZOIC  ERA.    ORIGIN  OF  PROTOPLASM  AND  OF  SIMPLEST  LIFE. 

COSMIC  TIME: 

FORMATIVE  ERA.    BIRTH  AND  GROWTH  OF  THE  EARTH  OUT  OF  THE 
SPIRAL  NEBULA  OF  THE  SUN. 

Beginnings  of  the  atmosphere  and  hydrosphere,  and  of  con- 
tinental platforms  and  oceanic  basins.  No  known  geological 
record. 

1  From  The  Earth's  Changing  Surface  and  Climate  by  Professor  Charles  Schuchert. 
See  Bibliography. 

358 


THE    ORIGIN    OF    SPECIES  359 

Even  a  casual  survey  of  this  history  - —  natural  history  — 
of  the  Earth  and  its  inhabitants  cannot  but  impress  one  with 
the  fact  that,  taken  all  in  all,  there  has  been  a  continuous, 
though  not  always  a  uniform,  advance  in  the  complexity  of 
organisms  from  the  most  ancient  times,  and  that  the  older 
types  seem  gradually  to  melt  into  modern  forms  as  the 
remoter  geological  eras  merge  into  the  more  recent.  "Only 
the  shortness  of  human  life  allows  us  to  speak  of  species  as 
permanent  entities."  Invertebrates  appear  in  the  Protero- 
zoic  Era;  Fishes  and  Amphibia  in  the  Paleozoic;  Reptiles, 
Birds,  and  Primitive  Mammals  in  the  Mesozoic;  higher 
Mammals  and  Man  in  the  Cenozoic.  Mosses  and  Ferns  arise 
before  Conifers  and  the  latter  before  the  familiar  Flowering 
Plants.  "Just  in  proportion  to  the  completeness  of  the 
geological  record  is  the  unequivocal  character  of  its  testimony 
to  the  truth  of  the  evolutionary  theory."  For  the  sake  of 
concreteness  we  may  select  two  examples  from  the  wealth  of 
material  offered  by  the  paleontologist. 

At  first  glance  there  seems  to  be  little  but  contrasts 
between  a  typical  Reptile  and  a  typical  Bird;  between  a 
cold-blooded,  scaly-skinned  Lizard,  let  us  say,  and  a  warm- 
blooded, feathered  Pigeon.  And  yet  the  zoologist  is  con- 
vinced that  Birds  have  evolved  from  a  reptilian  stock, 
because,  in  spite  of  superficial  dissimilarities,  there  are  funda- 
mental structural  resemblances  not  only  between  adult 
Reptiles  and  Birds,  but  also  between  their  developmental 
stages.  And  further,  because,  the  fossil  remains  of  a  very 
primitive  Bird,  Archaeopteryx,  have  been  found  which  form, 
in  many  ways,  a  connecting  link  between  the  Reptiles  and 
Birds  as  we  know  them  to-day. 

Archaeopteryx  was  undoubtedly  a  bird  about  the  size  of 
a  Pigeon,  but  one  with  jaws  supplied  with  many  small  teeth; 
with  a  long  lizard-like  tail  formed  of  many  vertebrae,  each 


360 


FOUNDATIONS   OF   BIOLOGY 


bearing  a  pair  of  quill  feathers;  w,ith  a  four-fingered  rep- 
tilian hand;  and  so  on.  In  brief,  just  such  a  creature  as  the 
imagination  of  an  evolutionist  would  picture  for  a  primitive 
Bird  has  been  disclosed  by  the  lithographic  stone  quarries  of 


Fia.  188.  —  Reptilian  Bird,  Arckaeopteryx,  (A),  compared  with  Pigeon, 
Columba  livia  (B).     (From  Lull.) 

Bavaria,  representing  the  later  Jurassic  period.     (Fig.  188.) 

The  ancestry  of  the  modern  Horse  has  been  the  most  im- 
pressive 'fossil  pedigree/  ever  since  Professor  Marsh  collected 
the  famous  series  of  fossil  skeletons  from  the  western  United 


THE    ORIGIN    OF    SPECIES  361 

States  and  arranged  them  in  the  Yale  Museum.  The  essen- 
tial facts  are  these.  Horse-like  animals  probably  arose 
from  an  extinct  group  known  as  the  Condylarthra-  which  had 
five  toes  on  each  foot  and  a  large  part  of  the  sole  resting 
on  the  ground.  However,  the  first  unquestionably  horse- 
like  form  found  in  North  America  is  a  little  animal  less  than 
a  foot  in  height,  known  as  Eohippus,  from  rocks  of  the  Eocene 
age.  The  fore-foot  of  Eohippus  has  four  complete  toes 
(digits  2,  3,  4,  and  5)  and  a  vestige  of  the  first  digit  in  the 
form  of  a  splint  bone.  The  hind-foot  has  three  toes  (digits 
2,  3,  and  4)  with  a  remnant  of  the  fifth  digit.  Later  in  the 
Eocene  we  find  Protorohippus  with  the  same  functional 
digits  but  lacking  the  vestiges.  Coming  to  the  Oligocene, 
Mesohippus  appears.  This  animal  is  about  the  size  of  a 
sheep  and  still  has  three  toes  (digits  2,  3,  and  4)  on  the  hind- 
foot,  but  only  three  complete  toes  (digits  2,  3,  and  4)  and  the 
vestige  of  a  fourth  (digit  5)  on  the  fore-foot.  Also  the 
middle  toe  (digit  3)  is  now  much  larger  than  the  side  toes, 
which  barely  touch  the  ground.  Then  during  the  late 
Miocene  and  early  Pliocene  we  find  Protohippus,  an  animal 
about  three  feet  tall,  with  three  toes  on  each  foot, but  with 
only  one  reaching  the  ground,  and  with  no  vestiges  of  other 
digits.  Finally,  toward  the  end  of  the  Pliocene,  appears 
the  genus  Equus  which  includes  the  modern  horse,  Equus 
caballus,  with  one  functional  toe  (digit  3)  on  each  foot  and 
the  remnants  of  two  more  (digits  2  and  4)  in  the  splint  bones. 
(Fig.  189.) 

In  this  outline  of  what  must  be  interpreted  as  the  fossil 
ancestors  of  the  Horse  of  to-day,  we  have  merely  selected 
several  representative  forms  to  emphasize  changes  in  foot 
structure.  But  the  reader  will  realize  that  many  other 
equally  significant  changes  were  involved  in  the  transforma- 
tion of  an  Eohippus  type  into  that  of  Equus.  This  much 


362 


FOUNDATIONS   OF   BIOLOGY 


THE    ORIGIN   OF    SPECIES 


363 


EVOLUTION   OF  THE   CAMELS 


•fe 
fe 


Recent 


3leistocene 


Auchenio 
(Llama) 


Skutl 


Feet 


1 


Pliocene 


Procamelus 


Miocene 


< 

fc 

^ 
d 


Poebrotherium 


Ohgocene 


Protylopus 


Eocene 


1 


Mesozoic  or  Age  of  Reptiles 


Hypothetical  five-toed  Ancestor 


FIG.  190.  —  Graphic  presentation  of  the  evolution  of  the  Camel. 
(From  Lull,  after  Scott.) 


364  FOUNDATIONS    OF   BIOLOGY 

appears  certain  to  the  biologist:  "In  early  Eocene  times 
there  lived  small  five-toed  hoofed  quadrupeds  of  generalized 
type,  that  the  descendants  of  these  were  gradually  specialized 
throughout  long  ages  along  similar  but  by  and  by  divergent 
lines,  that  they  lost  toe  after  toe  till  only  the  third  remained, 
that  they  became  taller  and  swifter,  that  they  gained  longer 
necks,  more  complex  teeth  and  larger  brains.  So  from  the 
short-legged  splay-footed  plodders  of  the  Eocene  marshes 
there  were  evolved  light-footed  horses  running  on  tiptoe 
on  the  dry  plains. "  (Thomson.) 

4.   Embryology 

If  evolution  is  a  fact,  one  would  expect  to  find  evidences 
of  the  genetic  relationships  of  organisms  in  their  individual 
development  from  egg  to  adult,  that  is  in  ontogeny.  Under 
former  headings  we  have  incidentally  mentioned  embryo- 
logical  data  which  point  toward  evolution,  so  that  now 
attention  may  be  confined  to  an  attempt  to  make  clear  a  fact 
of  first  importance — the  history  of  the  individual  (ONTOGENY) 
frequently  corresponds  in  broad  outlines  to  the  history  of  the 
race  (PHYLOGENY)  as  indicated  by  evidence  from  comparative 
anatomy,  etc.  If  we  have  in  mind  the  earlier  discussion  of 
Vertebrate  anatomy,  one  or  two  examples  will  suffice  to 
suggest  the  type  of  evidence  which  supports  this  so-called 

RECAPITULATION  THEORY,  Or  BIOGENETIC  LAW. 

Lower  Vertebrates,  such  as  the  Fishes,  have  a  heart  com- 
posed of  two  chief  chambers :  an  auricle  which  receives  blood 
from  the  body  as  a  whole  and  a  ventricle  which  pumps  it  to 
the  gills  on  its  way  to  supply  all  parts  of  the  body.  Among 
the  members  of  the  next  higher  group,  the  Amphibia  (Frogs, 
Toads,  etc.),  the  auricle  is  divided  into  two  parts,  while  the 
ventricle  remains  as  before.  Thus  these  forms  have  a  three- 
chambered  heart.  Passing  to  the  Reptiles,  we  find  that 


THE   ORIGIN    OF   SPECIES  365 

most  of  the  Lizards,  Snakes,  and  Turtles  have  the  ventricle 
partially  divided  into  two  chambers,  while  the  more  special- 
ized Crocodiles  and  Alligators  have  a  complete  partition  and 
therefore  a  four-chambered  heart.  This  is  the  condition  in 
all  adult  Birds  and  Mammals,  but  the  significant  fact  is  that, 
in  the  development  of  the  heart  of  the  individual  Bird  and 
Mammal,  embryonic  stages  succeed  each  other  which  parallel 
in  a  general  though  remarkable  way  this  sequence  from  a 
two-chambered  to  a  four-chambered  condition  as  exhibited 
in  the  adults  of  the  lower  Vertebrates.  (Figs.  91,  92.) 

Or  take  the  development  of  the  brain  in  the  Vertebrate 
series.  Even  in  the  human  embryo  the  fundament  of  the 
brain  arises  by  simple  transformations  of  the  anterior  end  of 
the  neural  tube,  which  at  first  are  nearly  indistinguishable 
from  the  conditions  which  exist  in  the  lowest  Vertebrates. 
Then  the  changes  become  progressively  more  complex  along 
lines  broadly  similar  to  those  occurring  from  Fish  to  Mammal, 
until  finally  the  complex  human  brain  is  formed.  (Figs. 
104,  105.) 

The  same  picture  is  presented  by  a  study  of  the  develop- 
ment of  the  excretory  system,  the  reproductive  system 
(Fig.  97),  the  skull,  and  so  on.  One  cannot  avoid  the  fact 
that  the  organs  of  higher  animals  pass  through  develop- 
mental stages  which  correspond  with  the  adult  condition 
of  similar  organs  in  lower  forms.  The  correspondence  is  not 
exact,  to  be  sure,  but  it  is  not  an  exaggeration  to  say  that 
embryological  development  is  parallel  to  that  which  ana- 
tomical study  leads  us  to  expect.  A  knowledge  of  the 
anatomy  of  an  animal  actually  gives  a  sound  basis  of  facts 
from  which  to  predict  in  broad  outlines  its  embryological 
development.  (Fig.  191.) 

What  are  the  bearings  of  these  facts  on  the  evolution 
theory?  It  is  perfectly  logical  to  conclude  that  it  is  an 


366 


FOUNDATIONS   OF   BIOLOGY 


'architectural  necessity,'  let  us  say,  for  the  four-chambered 
heart  to  arise  from  a  two-  and  three-chambered  condition  — 
and  undoubtedly  if  this  were  the  only  example  of  'ontogeny 
repeating   phylogeny'    the    conclusion   might   be   justified. 
But  when  one  considers  the  widespread  general  correspond- 


FIG.  191.  —  Embryos  in  corresponding  stages  of  development.    A,  Fish  (Shark) ; 
B,  Bird;   C,  Man.   g,  gill  slits.     (From  Scott.) 

ence  of  the  developmental  stages  in  higher  forms  with  con- 
ditions as  they  exist  in  the  adults  of  lower  forms,  the  facts 
almost  overwhelmingly  force  us  to  go  further  and  conclude 
that  the  similarity  has  its  basis  in  inheritance,  in  actual 
blood  relationship  between  the  higher  and  lower  forms,  in 
descent  with  modification  —  evolution. 


THE    ORIGIN   OF   SPECIES  367 

5.   Physiology 

Fundamental  structural  similarities  throughout  a  series 
of  organisms  implies  fundamental  physiological  similarities  — 
structure  and  function  go  hand  in  hand,  each  being  an  expres- 
sion of  -the  other.  But  the  physiological  evidence  is  less 
readily  presented  in  brief  form,  so  we  may  confine  attention 
to  one  striking  example  on  the  borderline. 

It  has  been  known  for  a  long  time  that  there  are  important 
chemical  differences — not  determinable  by  ordinary  chemical 
analysis — between  the  blood  even  of  closely  related  species, 
because  the  transfusion  of  the  blood  of  one  species  into 
another  is  usually  attended  by  physiological  disturbances 
and  often  by  death.  It  has  been  found  by  innumerable 
transfusions  and  also  by  so-called  precipitation  tests  of 
the  blood  in  vitro,  that  is  outside  the  body,  that  the  degree 
of  the  '  reaction '  is  in  many  cases  proportional  to  the  degree 
of  relationship  of  the  species  involved,  as  indicated  by  their 
classification  on  the  basis  of  anatomical  criteria. 

Thus,  as  one  would  expect,  human  blood  shows  closer 
chemical  relationships  with  the  blood  of  the  Man-like  Apes 
than  it  does  with  that  of  the  Old  World  Monkeys;  closer 
relationships  with  the  blood  of  the  latter  than  it  does  with 
that  of  the  New  World  Monkeys;  and  closer  with  the  blood  of 
these  than  with  that  of  the  Lemurs;  and  so  on.  Or,  descend- 
ing to  the  Reptiles:  paleontology  indicates  that  there  is  a 
close  relationship  between  Lizards  and  Snakes  and  also 
between  Turtles  and  Crocodiles,  while  the  reptilian  ancestor 
of  the  Birds  was  probably  more  closely  allied  with  the  latter 
than  the  former  groups.  These  same  relationships  are 
indicated  by  blood  tests. 

Thus  aside  from  a  few  startling  exceptions,  which  further 
study  perhaps  may  bring  into  line,  all  the  data  warrant  the 
conclusion  that  the  chemical  similarities  of  the  blood  are 


368  FOUNDATIONS   OF   BIOLOGY 

almost  as  constant  as  the  structural  similarities  of  the  blood 
vessels,  or,  in  evolutionary  terms,  "a  common  property  has 
persisted  in  the  bloods  of  certain  groups  of  animals  through- 
out the  ages  which  have  elapsed  during  their  evolution  from 
a  common  ancestor."  Blood  relationship  is  a  fact. 

6.   Distribution 

Every  one  recognizes  that  the  fauna  and  flora  are  not  the 
same  in  all  regions  of  the  Earth.  There  is  a  characteristic 
life  on  mountain,  plain,  and  seashore,  and  in  the  sea — as  well 
as  in  pond  and  puddle  —  and  also  in  the  Arctic,  Temperate, 
and  Torrid  zones.  But  the  problem  of  animal  and  plant 
distribution  is  by  no  means  so  simple  as  this  statement 
might  seem  to  imply,  because  the  study  involves  the  in- 
vestigation of  both  the  relations  of  the  various  organisms  to 
the  general  environing  conditions,  and  also  the  interrelations 
of  the  species  with  each  other.  It  forms  a  part  of  the  sciences 
of  plant  and  animal  ECOLOGY. 

Confining  attention  merely  to  the  geographical  distribu- 
tion of  animals  —  which  forms  the  science  of  ZOOGEOG- 
RAPHY—  let  us  take  a  couple  of  clear-cut  examples  and  see 
whether  special  creation  or  evolution  is  the  more  reasonable 
explanation  of  the  facts. 

At  the  present  time  a  characteristic  family  of  Mammals, 
known  as  the  Tapirs,  is  represented  by  distinct  species  in  two 
widely  separated  regions,  Central  and  South  America  and 
Southern  Asia  and  adjacent  islands.  But  paleontological 
studies  show  that  in  the  Pliocene  period  Tapirs  were  distrib- 
uted over  nearly  all  of  North  America,  Europe,  and  Northern 
Asia,  and  thereafter  gradually  became  extinct  so  that  by  the 
close  of  the  Pleistocene  period  the  remnants  were  distributed 
as  we  find  them  to-day.  In  brief,  the  present  discontinuous 
distribution  represents  the  remnants  of  a  world-wide  Tapir 


THE    ORIGIN    OF   SPECIES  369 

population,  and  the  differences  between  the  existing  species 
are  such  as  one  might  expect  to  find  among  the  members  of  a 
genus  long  isolated  in  different  environments  by  geographical 
barriers.  We  know,  for  example,  that  a  litter  of  European 
Rabbits  was  introduced  on  the  small  island  of  Porto  Santo 
during  the  fifteenth  century  and  by  the  middle  of  the  last 
century  its  descendants  had  become  so  distinct  from  the 
parent  form  that  it  was  described  as  a  'new  species/ 

As  a  matter  of  fact  the  characteristic  fauna  of  islands 
was  what  impressed  Darwin  with  the  need  of  some  interpre- 
tation other  than  special  creation.  During  his  famous  three 
years'  voyage  around  the  world  on  the  "Beagle,"  he  stopped 
at  the  Galapagos  Islands,  situated  about  600  miles  off  the 
west  coast  of  South  America,  and  was  astonished  to  find  that 
although  the  fauna  as  a  whole  resembled  fairly  closely  that 
of  the  mainland,  nevertheless  the  species  for  the  most  part 
not  only  were  different,  but  even  those  of  the  separate  islands 
were  distinct  —  the  islands  nearest  to  each  other  having 
species  most  similar.  Darwin  wrote,  "My  attention  was 
first  thoroughly  aroused  by  comparing  together  the  numerous 
specimens,  shot  by  myself  and  several  others  on  board,  of 
Mocking  Thrushes,  when,  to  my  astonishment,  I  discovered 
that  all  those  from  Charles  Island  belonged  to  one  species 
(Mimus  trifasdatus) ;  all  from  Albemarle  Island  to  M.  par- 
vulus]  and  all  from  James  and  Chatham  Islands  (between 
which  two  other  islands  are  situated  as  connecting  links) 
belonged  to  M .  melanotis." 

Darwin's  observations  of  such  facts  as  these  have  been 
corroborated  in  the  Galapagos  and  extended  to  isolated 
island  faunas  and  floras  all  over  the  world.  And  further,  his 
explanation  of  the  phenomena  is  the  most  plausible  extant. 
Continental  islands  secure  their  life  from  the  mainland  before 
they  are  cut  off,  and  Oceanic  islands  after  their  formation  by 


370 


FOUNDATIONS   OF   BIOLOGY 


Loxodon 
Africa 


Elephas 
Asia 


o 


Extinct          Nf 
Elephas 


Extinct 


Elephas       Mastodon 
? 


Extinct 
Dibelodon 


Extinct 

Meritherium 

Africa 


Meritherium 
Africa 


..t 


Proboscideo-Sirenian 
ancestor 


FIG.  192.  —  Chart  of  the  phylqgeny  of  the  Elephants,  showing  their  geological 
and  geographical  Hiytribution.      (After  T,ulM 


THE    ORIGIN    OF   SPECIES 


371 


FIG.  193.  —  Evolution  of  the  head  and  molar  teeth  of  Elephants.  A,  A',  Elephas, 
Pleistocene;  B,  Stegodon,  Pliocene;  C,  C',  Mastodon.  Pleistocene;  D,  D',  Trilophodon, 
Miocene;  E,  E',  Palaeomastodon,  Oligocene;  F,  F',  Moeritherium,  Eocene.  (After 
Lull.) 


372  FOUNDATIONS   OF   BIOLOGY 

volcanic  action  alone  or  aided  by  coral  growth.  In  either 
event  the  organisms  are  isolated  from  the  main  stock  of  the 
species,  and  in  proportion  to  the  length  of  time  and  the  degree 
of  isolation  the  insular  forms  diverge  until  separate  races  and 
species  arise.  Each  species  peculiar  to  each  isolated  island 
has  not  arisen  by  a  special  act  of  creation  but  by  descent  with 
modification. 

B.   FACTORS  OF  ORGANIC  EVOLUTION 

We  have  now  summarized  a  few  concrete  examples  of  the 
chief  types  of  evidence  that  organisms  —  species  —  have 
come  to  be  what  they  are  to-day  through  a  long  process  of 
descent  with  modification.  This  evidence,  taken  with  that 
presented,  so  to  speak,  on  and  between  the  lines  throughout 
this  work,  should-  place  the  reader  in  a  position  to  form  a  more 
or  less  independent  judgment  of  the  question.  It  is  only  nec- 
essary to  remind  him  again  that,  since  the  evidence,  from  the 
nature  of  the  case,  must  inevitably  be  indirect,  its  cogency  is 
tremendously  increased  by  its  amount.  And  the  overwhelm- 
ing impressiveness  of  all  the  concordant  evidence  for  organic 
evolution  the  reader,  with  only  a  very  limited  amount  of 
the  data  before  him,  cannot  appreciate. 

Taking  for  granted  the  fact  of  evolution  —  as  we  have  had 
to  do  throughout  —  what  are  the  factors  which  have  brought 
evolution  about?  That  is  quite  a  different  question,  but  one 
which  has  often  brought  confusion  to  the  popular  mind. 
Biologists  are  not  so  sure  to-day  as  they  were  a  generation 
ago  that  they  know  just  what  the  factors  are.  And  the  lay- 
man has  mistaken  their  questioning  of  one  factor  or  another 
for  a  questioning  of  the  fact. 

No  purpose  will  be  served  by  a  long  historical  account  of 
the  origin  of  the  present-day  point  of  view.  Suffice  it  to  say 
that  the  evolution  idea  is  a  generalization  which  has  crept 


THE    ORIGIN    OF    SPECIES 


373 


FIG.  194.  —  A  few  varieties  of  domestic  Pigeons.  Over  one  hundred  and  fifty 
different  breeds  have  been  derived  by  selection  from  the  wild  Blue-rock  Pigeon,  some 
of  which  "differ  fully  as  much  from  each  other  in  external  characters  as  do  the  most 
distinct  natural  genera."  (Darwin.)  1,  Blue-rock  Pigeon,  Columba  livia,  ancestral 
form;  2,  homing;  3,  common  mongrel;  4,  archangel;  5,  tumbler;  6,  bald-headed 
tumbler;  7,  barb;  8,  pouter;  9,  Russian  trumpeter;  10,  fairy  swallow;  11,  black- 
winged  swallow;  12,  fantail;  13,  carrier;  14,  15,  bluetts;  bird  between  14  and  15,  a 
tailed  turbit.  (From  photograph  of  an  exhibit  in  the  United  States  National  Museum.) 


374  FOUNDATIONS   OF   BIOLOGY 

from  science  to  science  —  from  Astronomy  to  Geology,  from 
Geology  to  Biology.  The  idea  in  one  form  or  another  is  as 
old  as  history,  but  for  all  practical  purposes  the  biologist 
Lamarck,  during  the  early  part  of  the  nineteenth  century, 
formulated  the  first  consistently  worked  out  theory  of  organic 
evolution.  But  the  evidence  he  presented  was  in  many  cases 
neither  happily  selected  nor  convincingly  presented  and  it 
was  laughed  out  of  court  by  biologists  and  laymen  alike. 
Lamarck's  evolution  factor  was  essentially  the  change  of  the 
organism  through  the  use  and  disuse  of  parts,  the  physiologi- 
cal response  of  the  organism  to  new  needs  offered  by  new  con- 
ditions of  life.  And  these  changes,  somatic  in  origin,  he  be- 
lieved were  transmitted  to  the  progeny.  As  we  know,  to- 
day little  or  no  value  is  placed  on  such  somatic  changes 
as  evolution  factors,  because  there  is  no  evidence  that  they 
are  heritable.  But  this  weak  point  was  not  the  one  which 
caused  the  rejection  of  the  theory  by  Lamarck's  contempo- 
raries. The  various  antagonistic  influences  can  be  summed 
up  by  saying,  the  time  was  not  ripe  for  evolution. 

Then  a  generation  later  appeared  Charles  Darwin  in 
England.  With  a  better  background  prepared  for  him  through 
the  headway  being  made  by  the  evolution  theory  in  geology, 
he  did  two  things.  He  presented  an  overwhelming  mass  of 
data  which  could  be  explained  most  reasonably  by  assuming 
the  origin  of  existing  species  by  descent  with  modification 
from  other  species.  And  he  offered  as  an  explanation  of  the 
origin  of  species  the  theory  of  "NATURAL  SELECTION,  or  the 
preservation  of  favoured  races  in  the  struggle  for  life."  It 
was  the  combination  of  the  facts  and  the  theory  to  account  for 
the  facts  which  won  the  thinking  world  to  organic  evolution. 

What,  in  brief,  was  the  theory?  In  the  first  place,  without 
discussing  the  cause  of  variations,  Darwin  showed  the  great 
amount  of  variation  in  nature.  And  any  and  all  kinds  of 


THE    ORIGIN   OF   SPECIES  375 

variation  were,  broadly  speaking,  equally  important  —  he 
made  no  sharp  distinction  between  somatic  and  germinal. 
The  universality  of  variations  established,  Darwin  empha- 
sized the  fact  that  the  power  of  reproduction  of  organisms  far 
exceeds  space  for  them  to  live  in  and  food  for  them  to  eat. 
Some  recent  facts  will  illustrate  this  point.  A  single  micro- 
scopic Paramecium  possesses  the  potentiality  to  eat,  grow, 
and  reproduce  —  to  transform  the  materials  of  its  environ- 
ment into  Paramecium  protoplasm  —  at  the  rate  of  3000 
generations  in  five  years.  And  all  the  descendants  (if  they 
actually  existed)  would  equal  2  raised  to  the  3000th  power, 
or  a  volume  of  protoplasm  approximately  equal  to  101000 
times  the  volume  of  the  Earth ! 

Something  must  inhibit  the  inherent  power  of  each  species 
to  overpopulate  the  Earth,  and  Darwin  emphasized  the 
struggle  for  existence  between  the  individuals  of  species. 
Since  the  struggle  is  so  keen,  a  variation,  however  slight, 
which  fits  —  adapts  —  an  individual  better  to  its  surroundings 
than  its  neighbors  are  adapted,  will,  more  often  than  not,  give 
its  possessor  an  advantage  in  the  struggle,  and  accordingly 
the  latter  will  survive  to  pass  on  the  favorable  variation  to 
its  progeny.  Thus  is  brought  about,  in  Spencer's  phraseology, 
"the  survival  of  the  fittest"  -the  survival  of  those  indi- 
viduals, and  therefore  species,  which  are  adapted  to  the  pe- 
culiar conditions  of  their  environment  and  mode  of  life.  And 
note,  this  offers  an  explanation  of  the  fact  of  adaptation  itself 
-  the  most  striking  phenomenon  which  organisms  exhibit. 

This  is  all  so  simple  from  one  point  of  view  and  so  confus- 
ingly  complex  from  others  that  it  may  well  be  restated  in  a 
couple  of  sentences  by  Darwin  himself:  "As  many  more 
individuals  of  each  species  are  born  than  can  possibly  survive, 
and  as,  consequently,  there  is  frequently  recurring  struggle 
for  existence,  it  follows  that  any  being,  if  it  vary  however 


376  FOUNDATIONS   OF   BIOLOGY 

slightly  in  any  manner  profitable  to  itself,  under  the  complex 
and  sometimes  varying  conditions  of  life,  will  have  a  better 
chance  of  surviving,  and  thus  be  naturally  selected.  From 
the  strong  principle  of  inheritance  any  selected  variety  will 
tend  to  propagate  its  new  and  modified  form." 

Nothing  succeeds  like  success,  and  once  started  Darwin's 
theory  gradually  swept  all  opposition  away,  and  some  of  its 
exponents  out-Darwined  Darwin.  Then,  as  was  to  be  ex- 
pected, the  reaction  came.  Acquired  characters  are  not 
heritable;  variations  are  swamped  by  interbreeding;  large 
variations  and  not  small  fluctuating  variations  are  crucial; 
and  so  on.  But  it  is  not  necessary  to  obscure  the  main  issue 
by  entering  into  these  controversies.  What  is  the  status  of 
the  theory  of  natural  selection  to-day? 

Evolution  is  not  a  closed  book  —  an  event  which  has  been 
completed  in  the  past  —  but  a  process  which  is  actively  going 
on  now.  "Nothing  endures  save  the  flow  of  energy  and  the 
rational  order  that  pervades  it."  And  there  is  every  reason 
to  believe  that  the  factors  involved  in  present  evolution  are 
the  same  as  those  which  have  operated  in  the  past.  The 
uniformitarian  doctrine  has  proved  productive  in  explain- 
ing the  evolution  of  the  Earth,  and  there  is  every  reason  to 
think  that  this  viewpoint  will  prove  —  is  proving  —  equally 
valuable  in  understanding  the  origin  of  the  diverse  inhabit- 
ants of  the  Earth.  We  have  come  to  realize  that  evolution 
is  a  bird's-eye  view  of  the  results  of  heredity,  since  the  origin 
of  life  —  the  facts  of  inheritance  hold  the  key  to  the  factors 
of  evolution.  Therefore  in  a  previous  chapter  we  discussed 
the  relations  of  recent  discoveries  in  genetics  to  the  evolution 
problem,  some  of  which  may  be  restated  now  with  special 
reference  to  the  origin  of  the  fitness  of  organisms. 

In  the  first  place  we  have  seen  that  though  variations  are 
the  rule  and  not  the  exception,  some  are  of  importance  for 


THE    ORIGIN    OF   SPECIES  377 

evolution  and  some  are  not.  All  the  evidence  indicates  that  the 
effective  variations  are  germinal  and  not  somatic.  Changes 
arising  in  the  soma  —  acquired  characters  —  are  unable 
so  specifically  to  modify  the  germ  that  they  are  'born  again' 
and  evolution  must  be  brought  about  by  the  evolution  of  the 
germ  itself.  Accordingly  selection  must  operate  to  eliminate 
the  'unfit'  germ  plasm  rather  than  the  unfit  soma,  though  as 
a  matter  of  fact  the  fitness  of  an  individual  is  determined 
largely  by  its  somatic  characters.  This  apparently  is  the 
crux  of  the  matter  and  presents  a  complication  of  the  mental 
picture  of  the  operations  of  selection  which  did  not  exist 
when  we  thought  of  soma  producing  germ  and  germ  produc- 
ing soma  again.  Since  individuals  frequently  belie  their 
germinal  condition  —  what  they  will  pass  on  to  their  progeny 
—  selection  has,  so  to  speak,  a  more  devious  though  not  less 
sure  path. 

Secondly,  how  does  the  germ  plasm  change?  Developed 
characters  are  the  result  of  the  activities  of  one  or  more  genes. 
Of  course,  a  gene  is  not  a  character.  It  is  not  even  an  unde- 
veloped character.  Characters  in  many  cases  arise  from  the 
interaction  of  several  genes,  though,  since  one  gene  deter- 
mines whether  the  gene  complex  will  give  rise  to  a  certain 
character,  it  is  really  the  determining  factor  —  for  example, 
the  so-called  sex  gene  on  the  so-called  sex  chromosome.  Such 
being  the  case,  characters  may  be  changed  by  alterations  of 
the  gene  complex.  (Cf.  p.  298.)  This  may  be  from  the 
influence  of  changes  within  the  soma  itself  or  from  the  en- 
vironment of  the  organism,  but  here  particularly  we  are 
on  debated  ground.  On  the  whole,  it  may  be  said  that  muta- 
tions—  germinal  changes  other  than  those  arising  from 
recombinations  —  seem  to  be  infrequent  compared  with 
non-heritable  changes  of  somatic  origin. 

These   facts   from   genetics,    taken    in    connection    with 


378  FOUNDATIONS   OF   BIOLOGY 

the  data  from  geographical  distribution,  the  succession  of 
types  in  the  geologic  past,  and  the  great  diversities  in  'breeds' 
in  nature,  etc.,  give  us  the  modern  background  for  attempt- 
ing to  form  an  opinion  of  the  method  of  evolution.  The 
consensus  of  opinion  seems  to  be  that  natural  selection  in 
some  form  is  the  guiding  principle  in  the  establishment  of  the 
'adaptive  complexes'  of  organisms.  Evolution  is  the  result 
of  germinal  variations,  largely  independent  of  environing 
conditions.  Many  of  these  variations  give  rise  to  characters 
which  neither  increase  nor  decrease  the  adaptation  of  the 
organism,  and  consequently  are  neutral  from  the  standpoint 
of  its  survival.  With  regard  to  such  characters  natural 
selection  is  essentially  inoperative.  Other  germinal  variations 
arise  which  produce  adaptive  structures  and  here  natural 
selection  is  effective  —  it  sifts  them  out,  as  it  were,  from  the 
unadaptive  and  neutral  variations  and  in  this  way  makes 
possible  their  survival  value  in  the  struggle  for  existence. 

So,  it  will  be  noted,  this  is  essentially  a  clarified  Darwinism. 
Instead  of  all  variations  being  heritable  —  some  are  inherited 
and  some  are  not.  Instead  of  all  heritable  variations  being 
important — some  are  and  some  are  not.  The  important 
ones  are  the  heritable  adaptive  variations  and  these  form 
the  raw  materials  for  natural  selection.  Natural  selection 
is  still  the  only  natural  explanation  of  that  coordinated 
adaptation  which  pervades  every  form  of  life,  but  it  is  prob- 
able—  indeed,  positive — that  there  are  more  factors  involved 
than  are  dreamt  of  in  our  philosophy. 


CHAPTER  XX 
EPOCHS  IN  BIOLOGICAL  HISTORY 

History  must  convey  the  sense  not  only  of  succession  but 
also  of  evolution. 

SOME  knowledge  of  hunting,  agriculture,  and  husbandry 
was  one  of  the  early  acquirements  of  prehistoric  Man,  and 
at  the  dawn  of  history,  nearly  5000  years  ago,  systems  of 
medicine  apparently  found  a  place  in  Egyptian  and  Babylo- 
nian civilizations.  So,  on  the  practical  side,  biology  has  a  very 
ancient  beginning.  But  biology  as  the  science  of  life  in 
which  emphasis  is  transferred  to  the  philosophical  —  to  the 
study  of  vital  phenomena  for  their  own  sake  —  really  begins 
with  the  Greeks. 

Science  reaching  Greece  from  the  South  and  East  fell  upon 
fertile  soil,  and  in  the  hands  of  the  Hellenic  natural  philoso- 
phers was  transformed  into  coherent  systems  through  the 
realization  that  nature  works  by  fixed  laws  —  a  conception 
foreign  to  the  Oriental  mind  and  the  corner-stone  of  all 
future  scientific  investigation.  It  is  not  an  exaggeration 
to  say  that  to  all  intents  and  purposes  the  Greeks  laid  the 
foundations  of  the  chief  subdivisions  of  natural  science  and, 
specifically,  created  biology. 

A.   GREEK  AND  ROMAN  SCIENCE 

ARISTOTLE  (384-322  B.C.),  the  most  famous  pupil  of 
Plato  and  dissenter  from  his  School,  represents  the  high- 
water  mark  of  the  Greek  students  of  nature  and  is  justly 
called  the  Father  of  Natural  History.  Although  Aristotle's 

379 


380  FOUNDATIONS   OF   BIOLOGY 

contributions  to  biology  are  manifold,  perhaps  of  most 
significance  is  the  fact  that  he  took  a  broad  survey  of  the 
existing  data  and  welded  them  into  a  science.  He  did  this 
by  relying,  to  a  considerable  extent,  on  the  direct  study  of 
organisms  and  by  insisting  that  the  only  true  path  of  advance 
lies  in  accurate  observation  and  description.  The  observa- 


FIG.  195.  —  Aristotle. 

tional  method  and  its  very  modern  development,  the  labora- 
tory method  of  biological  study,  find  their  first  great  exponent 
in  Aristotle.  But  mere  observation  without  interpretation 
is  not  science.  Aristotle's  generalizations  based  on  the  facts 
accumulated  and  his  elaboration  of  broad  philosophical 
conceptions  of  organisms  give  to  his  biological  works  their 
perennial  significance. 

While  Aristotle's  biological  investigations  were  devoted 


EPOCHS   IN   BIOLOGICAL   HISTORY  381 

chiefly  to  animals,  his  pupil  and  co-worker,  THEOPHRASTUS 
(370-286  B.C.),  made  profound  studies  on  plants.  Theo- 
phrastus  not  only  laid  the  foundations  but  also  gave  sug- 
gestions of  much  of  the  superstructure  of  botany;  an  achieve- 
ment which  entitles  him  to  rank  as  "the  first  of  real  botanists 
in  point  of  time." 
Before  leaving  the  Greeks  we  must  mention  HIPPOCRATES 


FIG.  196.  —  Theophrastus  of  Eresus. 

(460-370  B.C.)  ,  the  Father  of  Medicine.  Writing  a  generation 
before  Aristotle,  at  the  height  of  the  Age  of  Pericles,  Hippo- 
crates crystallized  the  knowledge  of  medicine  into  a  science, 
dissociated  it  from  philosophy,  and  gave  to  physicians  "the 
highest  moral  inspiration  they  have." 

The  history  of  medicine  and  of  biology  as  a  so-called  pure 
science  are  so  inextricably  interwoven  that  consideration  of 
the  one  involves  that  of  the  other.  Indeed  the  physicians 
form  the  only  bond  of  continuity  in  biological  history  be- 


382  FOUNDATIONS    OF   BIOLOGY 

tween  Greece  and  Rome.  The  chief  interest  of  the  Romans 
lay  in  technology,  and  it  is  but  natural  that  the  practical 
advantages  to  be  gained  from  medicine  should  ensure  its 
advance.  As  it  happens,  however,  two  Greek  physicians 
were  destined  to  have  the  most  influence:  Dioscorides,  an 
army  surgeon  under  Nero,  and  Galen,  physician  to  the 
Emperor  Marcus  Aurelius. 

DIOSCORIDES  wrote  the  first  important  treatise  on  applied 
botany.  This  was  really  a  work  on  the  identification  of 
plants  for  medicinal  purposes  but,  gaining  authority  with 
age,  it  became  the  standard  'botany'  for  fifteen  centuries. 

GALEN  (131-201)  was  the  most  famous  physician  of  the 
Roman  Empire  and  his  voluminous  works  represent  both  a 
depository  for  the  anatomical  and  physiological  knowledge 
of  his  predecessors,  rectified  and  worked  over  into  a  system, 
and  also  a  large  amount  of  original  investigation.  Galen 
was  at  once  a  practical  anatomist  and  the  first  experimental 
physiologist,  inasmuch  as  he  described  from  dissections  and 
insisted  on  the  importance  of  vivisection  and  experiment. 
Galen  gave  to  medicine  its  standard  'anatomy'  and  'physi- 
ology' for  fifteen  centuries. 

Any  consideration  of  the  biological  science  of  Rome  would 
be  incomplete  without  a  reference  to  the  vast  compilation 
of  fact  and  fancy  indiscriminately  mingled  made  by  PLINY 
the  Elder  (23-79).  It  was  aside  from  the  path  of  biological 
advance,  but  long  the  recognized  Natural  History,  passing 
through  some  eighty  editions  after  the  invention  of  printing. 

B.   MEDIEVAL  AND  RENAISSANCE  SCIENCE 

For  all  practical  purposes  we  may  consider  that  biology 
at  the  decline  of  the  Roman  Empire  was  represented  by  the 
works  of  Aristotle,  Theophrastus,  Dioscorides,  Galen,  and 
Pliny.  Even  these  exerted  little  influence  during  the  Middle 


EPOCHS    IN   BIOLOGICAL   HISTORY  383 

Ages,  being  saved  from  total  loss  for  future  generations 
chiefly  by  Arabian  scientists,  and  the  monasteries  of  Italy 
and  Britain.  In  so  far 'as  science  was  taught  at  all,  it  was 
from  small  compilations  of  corrupt  texts  of  ancient  authors 
interspersed  with  anecdotes  and  fables.  Under  theological 
influence  there  arose  the  oft-quoted  PHYSIOLOGUS,  found  in 
many  forms  and  languages,  which  is  at  once  a  collection  of 
natural  history  stories,  and  a  treatise  on  symbolism  and  the 
medicinal  use  of  animals.  The  centaur  and  phoenix  take 
their  place  with  the  Frog  and  Crow  in  affording  illustrations 
of  theological  texts  and  in  pointing  out  more  or  less  evident 
morals. 

So  low  had  science  fallen  that  the  scientific  Renaissance 
may  be  said  to  owe  its  origin  to  the  revival  of  classical  learn- 
ing —  to  the  translation  and  study  of  the  writings  of  Aristotle, 
and  other  authors  we  have  mentioned.  These  were  so 
superior  to  the  existing  science  that,  in  accord  with  the  spirit 
of  the  time,  Aristotle  and  Galen  became  the  bible  of  biology. 
The  first  works  were  merely  commentaries  on  the  writings 
of  these  authors,  but  as  time  went  on  more  and  more  new 
observations  were  interspersed  with  the  old.  In  short,  the 
climax  of  the  scientific  Renaissance  involved  a  turning  away 
from  the  authority  of  Aristotle  and  an  adoption  of  the 
Aristotelian  method  of  observation  and  induction. 

Botany  was  the  first  to  show  visible  signs  of  the  awakening, 
probably  because  of  the  dependence  of  medicine  on  plant 
products.  "All  physicians  professed  to  be  botanists  and 
every  botanist  was  thought  fit  to  practice  medicine."  In 
the  HERBALS  published  in  Germany  during  the  sixteenth 
century  we  can  trace  the  evolution  of  plant  description 
and  classification  from  mere  annotations  on  the  text  of 
Dioscorides  to  well-illustrated  manuals  of  the  flora  of 
western  Europe. 


384 


FOUNDATIONS   OF   BIOLOGY 


During  the  same  century  zoology  made  abortive  attempts 
to  emerge  as  a  science,  but  the  less  immediate  utility  of  the 
subject,  combined  with  the  difficulty  of  collecting  material 
and  therefore  the  necessity  of  more  dependence  on  travelers' 
tales,  contributed  to  retard  its  advance.  One  group  of  natu- 
ralists, the  ENCYCLOPAEDISTS,  so-called  from  their  endeavor  to 


FIG.  197.  —  Andreas  Vesalius. 

gather  all  available  information  of  living  things,  attempted 
the  impossible.  Gleaning  from  the  ancients  and  adding  such 
materials  as  they  could  gather,  led  to  the  publication  of  huge 
volumes  of  fact  and  fiction  whose  value  bore  no  just  propor- 
tion to  the  vast  expenditure  of  labor  —  even  in  the  case  of 
the  best,  Gesner's  History  of  Animals. 

Although  GESNER  (1516-1565)  of  Switzerland  was  without 
doubt  the  most  learned  naturalist  of  the  period  and  probably 
the  best  zoologist  who  had  appeared  since  Aristotle,  the  direct 


EPOCHS   IN   BIOLOGICAL   HISTORY  385 

path  to  progress  was  blazed  by  men  whose  plans  were  less 
ambitious.  Contemporaries  of  Gesner,  who  confined  their 
treatises  to  special  groups  of  organisms  which  they  themselves 
investigated,  really  instituted  the  biological  monograph  which 
has  proved  to  be  the  effective  method  of  scientific  publica- 
tion. 

While  the  herbalists,  encyclopaedists,  and  monographers 
at  work  in  natural  history  were  making  brave  endeavors  to 
develop  the  powers  of  independent  judgment,  which  were 
oppressed  to  such  an  extent  during  the  Middle  Ages  that  the 
very  activity  of  the  senses  seemed  stunted,  the  emancipator 
of  biology  from  the  traditions  of  the  ancients  appeared  in  the 
Belgian  anatomist,  ANDREAS  VESALIUS  (1514-1564).  Dis- 
gusted with  the  anatomy  of  the  time,  which  consisted  almost 
solely  in  interpreting  the  works  of  Galen  by  reference  to  crude 
dissections  made  by  barbers'  assistants,  Vesalius  attempted 
to  place  human  anatomy  on  the  firm  basis  of  exact  observa- 
tion. The  publication  of  his  great  work  On  the  Structure  of 
the  Human  Body  made  the  year  1543  the  dividing  line  be- 
tween ancient  and  modern  anatomy,  and  thenceforth  ana- 
tomical as  well  as  biological  investigation  in  general  broke 
away  from  the  yoke  of  authority  and  men  began  to  trust 
their  own  eyes. 

The  work  of  Vesalius  was  on  anatomy,  and  physiology  was 
treated  somewhat  incidentally.  The  complementary  work  on 
the  functional  side  came  in  1628  with  the  publication  of  the 
epoch-making  monograph  on  the  Motion  of  the  Heart  and 
Blood  in  Animals  by  WILLIAM  HARVEY  (1578-1657)  of  Lon- 
don. No  rational  conception  of  the  economy  of  the  animal 
organism  was  possible  under  the  influence  of  Galenic  physi- 
ology, and  it  remained  for  Harvey  to  demonstrate  by  a 
series  of  experiments,  logically  planned  and  ingeniously 
executed,  that  the  blood  flows  in  a  circle  from  heart  back  to 


386 


FOUNDATIONS    OF   BIOLOGY 


heart  again,  and  thus  to  supply  the  background  for  a 
proper  understanding  of  the  dynamics  of  the  organism  as  a 
whole.  With  the  work  of  Vesalius  and  Harvey,  biologists  had 
again  laid  hold  of  the  great  scientific  tools  —  observation, 


FIG.  198.  —  William  Harvey. 

experiment,   and   induction  —  which   since  then  have  not 
slipped  from  their  grasp. 

C.   THE  MICROSCOPISTS 

Even  while  the  marshalling  of  accurate  descriptions  of 
plants  and  animals  was  getting  under  way,  and  the  study  of 
macroscopic  anatomy  and  physiology  was  making  rapid 
strides  forward,  an  event  occurred  which  was  destined  to 
make  possible  modern  biology.  This  was  an  adaptation  of 
the  principle  of  the  spectacles  —  the  invention,  probably  by 
Roger  Bacon,  of  the  simple  microscope.  Then,  came  the 


EPOCHS   IN   BIOLOGICAL   HISTORY 


387 


compound  microscope  as  a  development  of  the  telescope 
at  the  hands  of  Galileo  about  1610,  and  by  the  middle  of  the 
century  simple  and  compound  microscopes  were  being  made 
by  opticians  in  the  leading  centers  of  Europe. 

The  earliest  clear  appreciation  of  the  importance  of  study- 
ing nature  with  instruments  which  increase  the  powers  of 
the  senses  in  general  and  of  vision  in  particular  is  found  in 


FIQ.  199.  —  Antony  van  Leeuwenhoek. 

a  remarkable  book,  by  HOOKE  (1635-1703)  of  London,  pub- 
lished in  1665.  Using  his  improved  compound  microscope, 
Hooke  clearly  observed  and  figured  for  the  first  time  the 
"little  boxes  or  cells"  of  organic  structure,  and  his  use  of  the 
word  cell  is  responsible  for  its  application  to  the  protoplas- 
mic units  of  modern  biology. 

Microscopical  work  was  a  mere  incident  among  the  varied 
interests  of  Hooke,  while  LEEUWENHOEK  (1632-1723)  of 
Holland  spent  a  long  life  studying  nearly  everything  which 


388  FOUNDATIONS   OF   BIOLOGY 

he  could  bring  within  the  scope  of  his  simple  lenses.  With  an 
unexplored  field  before  him,  all  of  his  observations  were  dis- 
coveries. Bacteria,  Protozoa,  Hydra,  and  many  other 
organisms  were  first  revealed  by  his  lenses.  But  Leeuwen- 
hoek's  discovery  of  the  sperm  of  animals  created  the  most 
astonishment.  His  imagination,  however,  outstripped  his 
observations  for  he  thought  he  saw  evidence  of  the  organism 
preformed  within  the  sperm  and  so  came  to  regard  it  as  the 
true  germ  which  had  only  to  be  hatched  by  the  female. 

The  patience  of  Leeuwenhoek  would  have  been  strained 
to  the  breaking  point  by  the  studies  on  insect  anatomy  made 
by  SWAMMERDAM  (1637-1680)  of  Holland.  Instigated 
largely  by  the  desire  to  refute  the  current  notion  that  insects 
and  similar  lower  animals  are  without  complicated  internal 
organs,  Swammerdam  spent  his  life  in  studies  on  their  struc- 
ture and  life  histories.  Revealing,  as  he  did,  by  the  most  deli- 
cate technique  in  dissection,  the  finest  details  observable 
with  his  lenses,  Swammerdam  not  only  set  a  standard  for 
minute  anatomy  which  was  unsurpassed  for  a  century,  but 
also  dissipated  for  all  time  the  conception  of  simplicity  of 
structure  in  the  lower  animals.  He  thus,  quite  naturally, 
added  one  more  argument  to  those  of  the  Italian  REDI  (1626- 
1698)  and  others  against  spontaneous  generation. 

Malpighi  of  Bologna  and  Grew  of  London,  contemporaries 
of  Hooke,  Leeuwenhoek,  and  Swammerdam,  may  be  con- 
sidered as  the  pioneer  histologists.  GREW  (1641-1712) 
devoted  all  his  attention  to  plant  structure,  while  MALPIGHI 
(1628-1694),  in  addition  to  botanical  studies  which  paralleled 
Grew's,  made  elaborate  investigations  on  animals. 

The  versatility  as  well  as  the  genius  of  Malpighi  is  shown  by 
his  studies  on  the  anatomy  of  plants,  the  function  of  leaves, 
the  development  of  the  plant  embryo,  the  embryology  of  the 
chick,  the  anatomy  of  the  silkworm,  and  the  structure  of 


EPOCHS   IN   BIOLOGICAL   HISTORY  389 

glands.  Skilled  in  anatomy  but  with  prime  interest  in  physi- 
ology, his  lasting  contribution  lies  in  his  dependence  upon  the 
microscope  for  the  solution  of  problems  where  structure  and 
function,  so  to  speak,  merge.  This  is  well  illustrated  by  his 
ocular  demonstration  of  the  capillary  circulation  in  the  lungs, 
which  is  not  only  his  greatest  discovery  but  also  the  first 


FIG.  200.  —  Marcello  Malpighi. 

of  prime  importance  ever  made  with  a  microscope,   since 
it  completed  Harvey's  work  on  the  circulation  of  blood. 

D.    THE  DEVELOPMENT  OF  THE  SUBDIVISIONS  OF  BIOLOGY 

The  microscopists  taken  collectively  created  an  epoch  in 
the  history  of  biology,  so  important  is  the  lens  for  the  ad- 
vancement of  the  science.  Broadly  speaking,  we  find  that 
its  development  along  many  lines  during  the  eighteenth  and 
particularly  the  nineteenth  century  went  hand  in  hand  with 
improvements  in  the  compound  microscope  itself  and  in  mi- 
croscopical technique.  Again,  the  microscopists  in.  general 
and  Malpighi  in  particular  opened  up  so  many  new  paths  of 


390  FOUNDATIONS   OF   BIOLOGY 

advance  that  from  this  period  on  it  is  not  possible,  even  in 
the  most  general  survey,  to  discuss  the  development  of  biol- 
ogy as  a  whole.  The  composite  picture  must  be  formed  by 
emphasizing  and  piecing  together  various  lines  of  work,  such 
as  taxonomy,  comparative  anatomy  of  animals,  embry- 
ology, physiology  of  plants  and  animals,  genetics,  and  evolu- 
tion. 

1.     Taxonomy 

Taxonomy  has  as  its  object  the  bringing  together  of  or- 
ganisms which  are  alike  and  the  separating  of  those  which  are 
unlike;  a  problem  of  no  mean  proportions  when  a  conserva- 
tive estimate  to-day  shows  upward  of  a  million  species  of 
animals  and  plants  —  leaving  out  of  account  the  myriads 
of  forms  represented  only  by  fossil  remains. 

Naturally  the  earliest  classifications  were  utilitarian  or 
more  or  less  physiological,  but  as  knowledge  increased  em- 
phasis was  shifted  to  the  anatomical  criterion  of  specific  dif- 
ferences, and  thenceforth  classification  became  an  important 
aspect  of  natural  history  —  a  central  thread  both  practical 
and  theoretical.  Practical,  in  that  it  involved  the  arranging 
of  living  forms  so  that  a  working  catalog  was  made  which 
required  nice  anatomical  discrimination,  and  therefore  the 
amassing  of  a  large  body  of  facts  concerning  animals  and 
plants.  Theoretical,  because  in  this  process  botanists  and 
zoologists  were  impressed,  almost  unconsciously  at  first,  with 
the  'affinity'  of  various  types  of  animals  and  plants,  and  so 
were  led  to  problems  of  their  origin. 

From  Aristotle,  who  emphasized  the  grouping  of  organisms 
on  the  basis  of  structural  similarities,  we  must  pass  over  some 
seventeen  centuries,  in  which  the  only  work  of  interest  was 
done  by  the  herbalists  and  encyclopaedists,  to  the  time  of 
RAY  (1628-1705)  of  England  and  LINNAEUS  (1707-1778)  of 
Sweden.  Previous  to  Ray  the  term  species  was  used  some- 


EPOCHS    IN    BIOLOGICAL   HISTORY 


391 


what  indefinitely,  and  his  chief  contribution  was  to  make  the 
word  more  concrete  by  applying  it  solely  to  groups  of  similar 
individuals  which  seem  to  exhibit  constant  characters  from 
generation  to  generation.  This  paved  the  way  for  the  great 
taxonomist,  Linnaeus. 

Linnaeus  was  first  and  foremost  a  botanist  who  gave  plant 
students  at  once  a  practical  classification  of  Flowering  Plants, 


m 


FIG.  201.  —  Carolus  Linnaeus. 

based  chiefly  on  the  number  and  arrangement  of  the  stamens ; 
and  at  the  same  time  insisted  on  brief  descriptions  and  the 
scheme  of  giving  each  kind  of  organism  a  name  of  two  words, 
generic  and  specific,  thereby  establishing  the  system  of 
BINOMIAL  NOMENCLATURE.  Linnaeus'  success  with  botani- 
cal taxonomy  led  him  to  extend  the  principles  to  animals  and 
even  to  the  so-called  mineral  kingdom:  the  latter  showing 
at  a  glance  his  lack  of  appreciation  of  any  genetic  relation- 
ship between  species.  Although  the  terms  genus  and  species 
to  Linnaeus  expressed  a  transcendental  affinity,  since  he 


392 


FOUNDATIONS   OF   BIOLOGY 


believed  that  species,  genera,  and  even  higher  groups  repre- 
sented distinct  acts  of  creation,  nevertheless  his  greatest 
works,  the  Species  Plantarum  and  Systema  Naturae,  are  of 
outstanding  importance  in  biological  history  and  by  common 
consent  the  base  line  of  priority  in  botanical  and  zoological 
nomenclature. 

2.   Comparative  Anatomy 

Owing  to  the  less  marked  structural  differentiation  of 
plants  in  comparison  with  animals,  plant  anatomy  lends 


FIG.  202.  —  Georges  Cuvier. 

itself  less  readily  to  descriptive  analysis,  so  that  an  epoch  in 
the  study  of  comparative  anatomy  is  not  so  well  defined  in 
botany  as  in  the  sister  science,  zoology.  Therefore,  we  shall 
confine  our  attention  to  the  comparative  anatomy  of  animals. 
Comparative  anatomy  as  a  really  important  aspect  of 
zoological  work,  in  fact  as  a  science  in  itself,  was  the  result 
of  the  life-work  of  CUVIER  (1769-1832)  of  Paris.  It  is  true 


EPOCHS    IN    BIOLOGICAL   HISTORY  393 

that  some  of  his  predecessors  had  reached  a  broad  viewpoint 
in  anatomical  study,  but  Cuvier's  claim  to  fame  rests  on  the 
remarkable  breadth  of  his  investigations  —  his  grasp  of  the 
comparative  anatomy  of  the  whole  series  of  animal  forms. 
And  not  content  merely  with  the  living,  he  made  himself  the 


FIG.  203.  —  Thomas  Henry  Huxley. 

first  real  master  of  the  anatomy  of  fossil  Vertebrates  as  was 
his  contemporary,  Lamarck,  of  fossil  Invertebrates. 

Cuvier's  grasp  of  anatomy  was  due  to  his  emphasizing,  as 
Aristotle  had  done  before  him,  the  functional  unity  of  the  or- 
ganism: that  the  interdependence  of  organs  results  from  the 
interdependence  of  function:  that  structure  and  function  are 
two  aspects  of  the  living  machine  which  go  hand  in  hand. 
Cuvier's  famous  principle  of  correlation  —  "Give  me  a 
tooth,"  said  he,  "and  I  will  construct  the  whole  animal" 
is  really  an  outcome  of  this  viewpoint.  Every  change  of 


394  FOUNDATIONS    OF   BIOLOGY 

function  involves  a  change  in  structure  and,  therefore,  given 
extensive  knowledge  of  function  and  of  the  interdependence 
of  function  and  structure,  it  is  possible  to  infer  from  the 
form  of  one  organ  that  of  most  of  the  other  organs  of  an 
animal.  But  Cuvier  undoubtedly  allowed  himself  to  exagger- 
ate his  guiding  principle  until  it  exceeded  the  bounds  of  facts. 
Among  Cuvier 's  immediate  successors,  OWEN  (1804-1892) 
of  London  perhaps  demands  special  mention.  Owen  spent  a 
long  life  dissecting  with  untiring  patience  and  skill  a  remark- 
able series  of  animal  types,  as  well  as  reconstructing  extinct 
forms  from  fossil  remains.  Aside  from  the  facts  accumu- 
lated, probably  his  greatest  contribution  was  making  con- 
crete the  distinction  between  homologous  and  analogous 
structures,  which  has  been  of  the  first  importance  in  working 
out  the  pedigrees  of  plants  as  well  as  of  animals;  though  Owen 
himself  took  an  enigmatical  position  in  regard  to  organic 
evolution  —  quite  different  from  that  of  his  great  English 
contemporary  comparative  anatomist,  HUXLEY  (1825-1895). 

3.   Physiology 

The  functions  of  organisms  were  discussed  by  Aristotle 
with  his 'usual  insight,  though,  as  might  be  expected  since 
physiology  is  more  dependent  than  anatomy  upon  progress 
in  other  branches  of  science,  with  less  happy  results.  Simi- 
larly Galen  was  hampered  in  his  attempt  to  make  physiology 
a  distinct  department  of  learning,  based  on  a  thorough  study 
of  anatomy,  and  the  corner  stone  of  medicine;  though  fate 
foisted  upon  uncritical  generations  through  fifteen  centuries 
his  system  of  human  physiology.  The  worst  of  it  was  not 
that  it  was  nearly  all  wrong,  but  that  to  question  Galen's 
physiology  or  anatomy  became  little  less  than  sacrilege  until 
the  studies  of  Vesalius  and  Harvey  brought  a  realization  that 
Galen  had  not  quite  finished  the  work. 


EPOCHS    IN   BIOLOGICAL   HISTORY  395 

Neither  Vesalius  nor  Harvey  made  an  attempt  to  explain 
the  workings  of  the  body  by  appeal  to  so-called  physical 
and  chemical  laws;  and  for  good  reason.  Chemistry  had 
not  yet  thrown  off  the  shackles  of  alchemy  and  taken  its 
legitimate  place  among  the  elect  sciences,  while  during 
Harvey's  lifetime,  under  the  influence  of  Galileo,  the  new 
physics  was  born.  But  by  the  end  of  the  seventeenth 
century  both  physics  and  chemistry  had  forced  their  way  into 
physiology  and  split  it  into  two  schools.  The  physical  school 
was  founded  by  BORELLI  (1608-1679)  of  Italy,  who,  employ- 
ing incisive  physical  methods,  attacked  a  series  of  problems 
with  brilliant  results;  while  the  chemical  school  developed 
from  the  influence  of  FKANCTSCUS  SYLVIUS  (1614-1672)  of 
Holland  as  a  teacher  rather  than  as  an  investigator. 

This  awakening  brought  a  host  of  workers  into  the  field 
and  the  harvest  of  the  century  was  garnered  and  enriched 
by  HALLER  (1708-1777)  of  Geneva.  In  a  comprehensive 
treatise  which  at  once  indicated  the  erudition  and  critical 
judgment  of  its  author,  Haller  established  physiology  as  a 
distinct  and  important  branch  of  biological  science.  It  was 
no  longer  a  mere  adjunct  of  medicine.  Perhaps  the  most 
significant  advance  in  Haller's  century  consisted  in  setting 
the  physiology  of  nutrition  and  of  respiration  —  both  of 
which  awaited  the  work  of  the  chemists  —  well  upon  the  way 
toward  their  modern  form. 

REAUMUR  (1683-1757)  of  Paris  and  SPALLANZANI  (1729- 
1799)  of  Pavia  may  be  singled  out  for  their  exact  studies  of 
gastric  digestion,  which  showed  solution  of  the  food  to  be 
the  main  factor  in  digestion  —  though  it  was  not  clear  how 
these  changes  differ  from  ordinary  chemical  ones.  It  was 
left  for  nineteenth-century  investigators  to  establish  the  fact 
that  food  in  passing  along  the  digestive  tract  runs  the  gauntlet 
of  a  series  of  complex  chemical  substances,  each  of  which  has 


396  FOUNDATIONS   OF   BIOLOGY 

its  part  to  play  in  putting  the  various  constituents  of  the  food 
into  such  a  form  that  they  can  pass  to  the  various  cells  of 
the  body  where  they  are  actually  used. 

On  the  side  of  respiration,  a  closer  approach  was  made 
toward  a  true  understanding  of  the  process.  In  France 
LAVOISIER  (1743-1794)  made  it  clear  that  the  chemical 
changes  taking  place  in  respiration  involve  essentially  a 
process  of  combustion,  and  it  only  remained  for  later  work 
to  show  that  this  takes  place  in  the  tissues  rather  than  in 
the  lungs. 

Enough  perhaps  has  been  said  to  indicate  the  trend  of 
physiology  away  from  the  maze  of  Galenic  "spirits"  in 
which  science  lost  itself,  toward  the  modern  viewpoint  of 
science  which  assumes  as  its  working  hypothesis  that  life  phe- 
nomena are  an  expression  of  a  complex  interaction  of  physico- 
chemical  laws  which  do  not  differ  fundamentally  from  the 
so-called  laws  operating  in  the  inorganic  world,  and  that  the 
economy  of  the  organism  is  in  accord  with  the  law  of  the 
conservation  of  energy  —  probably  the  most  far-reaching 
generalization  attained  by  science  during  the  past  century. 

Most  of  the  firm  foundation  on  which  the  physiology  of 
animals  rests  to-day  has  been  built  up  by  the  work  on  Verte- 
brates. But  since  the  middle  of  the  nineteenth  century, 
when  the  versatile  MULLER  (1801-1858)  of  Germany  empha- 
sized the  value  of  studying  the  physiology  of  higher  and  lower 
animals  alike,  there  has  been  an  ever-increasing  tendency 
to  focus  evidence,  in  so  far  as  possible,  from  all  forms  of  life 
on  general  problems  of  function.  This  Jias  culminated  in  the 
science  of  COMPARATIVE  PHYSIOLOGY. 

The  less  obvious  structural  and  functional  differentiation 
of  plants  retarded  progress  in  plant  physiology  as  it  did  in 
plant  anatomy.  Probably  of  most  historical  and  certainly 
of  most  general  interest  is  the  development  of  our  knowledge 


EPOCHS   IN   BIOLOGICAL  HISTORY  397 

of  the  nutrition  of  green  plants.  Aristotle's  notion  that 
the  plant's  food  is  prepared  for  it  in  the  ground  was  still 
prevalent  during  the  seventeenth  century  when  Malpighi, 
from  his  studies  on  plant  histology,  gave  the  first  hint  of 
supreme  importance  —  the  crude  sap  enters  by  the  roots 
and  is  carried  to  the  leaves  where,  by  the  action  of  sunlight, 
evaporation,  and  some  sort  of  a  fermentation,  it  is  elaborated 
and  distributed  as  food  to  the  plant  as  a  whole. 


Fio.  204.  —  Stephen  Hales. 

It  is  STEPHEN  HALES  (1677-1761)  of  England,  however,  to 
whom  the  botanist  looks  as  the  Harvey  of  plant  physiology, 
because  in  his  Vegetable  Statics  (1727)  he  laid  the  foundations 
of  the  physiology  of  plants  by  making  "  plants  speak  for 
themselves"  through  his  incisive  experiments.  For  the 
first  time  it  became  clear  that  green  plants  derive  a  con- 
siderable part  of  their  food  from  the  atmosphere,  and  also 
that  the  leaves  play  an  active  role  in  the  movements  of 
fluids  up  the  stem  and  in  eliminating  superfluous  water  by 
evaporation.  Still  the  picture  was  incomplete,  and  so  it 


398  FOUNDATIONS   OF   BIOLOGY 

remained  until  the  biologist  had  recourse  to  further  data 
from  the  chemist,  in  1779,  PRIESTLEY  (1733-1804)  of  Eng- 
land, the  discoverer  of  oxygen,  showed  that  this  gas  under 
certain  conditions  is  liberated  by  plants.  This  fact  was 
seized  upon  by  a  native  of  Holland,  INGENHOUSZ  (1730- 
1799),  who  demonstrated  that  carbon  dioxide  from  the  air 
is  reduced  to  its  component  elements  in  the  leaf  during 
exposure  to  sunlight.  The  plant  retains  the  carbon  and 
returns  the  oxygen  —  this  process  of  carbon-getting  being 
quite  distinct  from  that  of  respiration  in  which  carbon 
dioxide  is  eliminated.  It  remained  then  for  DE  SAUSSURK 
(1767-1845)  in  Geneva  to  show  that,  in  addition  to  the 
fixation  of  carbon,  the  elements  of  water  are  also  employed, 
while  from  the  soil  various  salts,  including  combinations  of 
nitrogen,  are  obtained.  But  it  was  nearly  the  middle  of 
the  last  century  before  the  influence  and  work  of  LIEBIG 
(1803-1873)  at  Giessen  led  to  a  clear  realization  of  the 
fundamental  part  played  by  the  chlorophyll  of  the  green 
leaf  in  making  certain  chemical  elements  available  to  animals. 
The  establishment  of  the  cosmical  function  of  green  plants  — 
the  link  they  supply  in  the  circulation  of  the  elements  in 
nature  —  is  a  landmark  in  biological  progress. 

4.   Histology 

Studies  on  the  physiology  of  plants  and  animals  natural^ 
involved  the  progressive  analysis  of  the  physical  basis  of 
the  phenomena  under  consideration,  but  the  Aristotelian 
classification  of  the  materials  of  the  body  as  unorganized 
substance,  homogeneous  parts  or  tissues,  and  heterogeneous 
parts  or  organs,  practically  represented  the  level  of  analysis 
until  the  beginning  of  the  eighteenth  century.  In  fact  it  was 
not  .until  the  revival  of  interest  in  embryology  early  in 
the  last  century  that  the  cell  became  a  particular  object  of 


EPOCHS    IN    BIOLOGICAL   HISTORY  399 

study,  and  attention  began  gradually  to  shift  from  more  or 
less  superficial  details  to  cell  organization.  This  culminated  in 
the  classic  investigations  of  two  German  biologists,  the 
botanist  SCHLEIDEN  (1804-1881)  and  the  zoologist  SCHWANN 
(1810-1882),  published  in  1838  and  1839.  Together  these 
studies  clearly  showed  that  all  organisms  are  composed  of 


FIG.  205.  —  Matthias  Jacob  Schleiden. 

units,  or  cells,  which  are  at  once  structural  entities  and  the 
centers  of  physiological  activities.  And  further  that  the 
development  of  animals  and  plants  consists  in  the  multi- 
plication of  an  initial  cell  to  form  the  multitude  of  different 
kinds  which  constitute  the  adult.  Unquestionably  the  cell 
concept  represents  one  of  the  greatest  generalizations  in 
biology,  and  it  only  needed  for  its  consummation  the  full 
realization  that  the  viscid,  jelly-like  material  which  zoolo- 


400 


FOUNDATIONS   OF   BIOLOGY 


gists  interpreted  as  the  true  living  matter  of  animals,  and  the 
quite  similar  material  which  botanists  considered  the  true 
living  part  of  plants  are  practically  identical.  This  viewpoint 
was  crystallized  in  the  early  sixties  by  SCHULTZE  (1825- 
1874)  of  Germany  in  the  formulation  of  the  protoplasm 
concept  and  thenceforth  not  only  morphological  elements  — 


FIG.  206.  —  Theodor  Schwann. 


cells  —  but  also  the  material  of  which  they  are  composed  — 
protoplasm  —  were  recognized  as  fundamentally  the  same  in 
all  living  beings.  Indeed,  the  realization  of  a  common  physi- 
cal basis  of  life  in  both  plants  and  animals  —  a  common 
denominator  to  which  all  vital  phenomena  are  reducible  — 
gave  content  to  the  term  biology  and  created  the  science  of 
life  in  its  modern  form. 


EPOCHS   IN   BIOLOGICAL   HISTORY  401 

5.   Embryology 

The  enunciation  of  the  cell  theory  came,  as  we  have  seen, 
from  combined  studies  on  the  adult  structure  and  on  the 
development  of  plants  and  animals  from  the  germ  or  egg,  and 
accordingly  implies  that  the  science  of  embryology  has  a 
history  of  its  own.  As  a  matter  of  fact,  Aristotle  discussed 
the  wonder  of  the  beating  heart  in  the  hen's  egg  after  three 
days'  incubation,  but  there  the  subject  rested  until  FABRICIUS 
( 1537-1 6 19)  at  Padua,  early  in  the  seventeenth  century,  pub- 
lished a  treatise  which  illustrated  the  obvious  sequence  of 
events  within  the  hen's  egg  to  the  time  of  hatching.  This  be- 
ginning was  built  upon  by  a  pupil  of  Fabricius,  the  cele- 
brated Harvey,  who  added  many  details  of  interest,  though 
little  progress  in  embryology  was  possible  without  the  micro- 
scope. This  was  first  turned  on  the  problem  by  the  versatile 
Malpighi  in  two  treatises  published  in  1672,  and  at  one  step 
animal  development  was  placed  upon  a  plane  so  advanced 
that  for  over  a  century  it  was  unappreciated.  One  conclusion 
of  Malpighi,  however,  was  seized  upon  by  contemporary 
biologists.  Apparently,  unbeknown  to  him,  some  of  the  eggs 
which  be  studied  were  slightly  incubated,  so  that  he  thought 
traces  of  the  future  organism  are  preformed  in  the  egg.  This 
error  contributed  to  the  formulation  of  the  preformation 
theory,  which  gradually  became  the  dominant  question  in 
embryology. 

As  a  matter  of  fact  the  time  was  not  ripe  for  theories  of 
development.  The  preformationists  were  wrong,  but  so  were 
Aristotle,  Harvey,  and  others  who  went  to  the  opposite  ex- 
treme and  denied  all  egg  organization  and  therefore  tried  to 
get  something  out  of  nothing.  It  remained,  as  we  know,  for 
the  present  generation  of  embryologists  to  work  out  many  of 
the  details  of  the  origin  and  organization  of  the  germ  cells, 


402 


FOUNDATIONS   OF   BIOLOGY 


and  to  reach  a  level  of  analysis  deep  enough  to  suggest  how 
"the  whole  future  organism  is  potentially  and  materially  im- 
plicit in  the  fertilized  egg  cell"  and  thus  that  "the  preforma- 
tionist  doctrine  had  a  well-concealed  kernel  of  truth  within 
its  thick  husk  of  error." 

The  next  great  advance  came  in  the  accurate  and  compre- 
hensive studies  of  the  Russian,  VON  BAER  (1792-1876),  pub- 


FIG.  207.  —  Karl  Ernst  von  Baer. 

lished  in  the  thirties  of  the  last  century.  Taking  his  material 
from  all  the  chief  groups  of  higher  animals,  von  Baer  founded 
COMPARATIVE  EMBRYOLOGY.  Among  his  achievements  may  be 
mentioned :  the  clear  discrimination  of  the  chief  developmental 
stages,  such  as  cleavage  of  the  egg,  germ  layer  formation,  tis- 
sue and  organ  differentiation;  the  insistence  on  the  importance 
of  the  facts  of  development  for  classification;  and  the  dis- 
covery of  the  egg  of  Mammals.  His  observations  on  the 


EPOCHS   IN    BIOLOGICAL  HISTORY  403 

origin  and  development  of  the  germ  layers,  which  afforded 
the  key  to  many  general  problems  of  the  origin  of  the  body- 
form  (morphogenesis) ,  and  his  emphasis  on  the  resemblance  of 
certain  embryonic  stages  of  higher  animals  to  the  adult  stages 
of  lower  forms,  were  crystallized  by  his  successors,  under  the 
influence  of  the  evolution  theory,  as  the  germ  layer  theory 
and  the  recapitulation  theory. 

From  every  point  of  view  von  Baer  created  an  epoch  in 
embryology  synchronous  with  the  formulation  of  the  cell 
theory  by  Schleiden  and  Schwann,  and  it  thenceforth  became 
the  problem  of  the  embryologist  to  interpret  development  in 
terms  of  the  cell.  It  is  unnecessary  to  follow  historically  the 
establishment  of  the  fact  that  the  egg  and  the  sperm  are 
really  single  nucleated  cells;  that  fertilization  consists  in  the 
fusion  of  egg  and  sperm  and  the  orderly  arrangement  of  their 
chief  nuclear  contents,  or  chromosomes;  that  the  new  genera- 
tion is  the  fertilized  egg,  since  every  cell  of  the  body  as  well  as 
every  chromosome  in  every  cell  is  a  lineal  descendant  by 
division  from  the  zygote,  and  so  from  the  gametes  which 
united  at  fertilization  to  form  it.  Such,  however,  are  the 
chief  results  of  cytological  study  since  von  Baer.  But  em- 
bryologists  have  not  been  content  to  employ  merely  the  de- 
scriptive method,  and  the  dominant  note  of  the  most  modern 
research  is  physiological  —  the  experimental  study  of  the 
significance  of  fertilization,  the  dynamics  of  cell  division,  the 
basis  of  differentiation,  the  influence  of  environmental 
stimuli,  and  so  on. 

6.   Genetics 

The  study  of  inheritance  could  be  little  more  than  a  grop- 
ing in  the  dark  until  embryology,  under  the  influence  of  the 
cell  theory,  afforded  a  body  of  facts  which  clearly  indicated 
that  typically  the  fertilized  egg  is  the  sole  bridge  of  continuity 


404  FOUNDATIONS   OF   BIOLOGY 

between  successive  generations.  Indeed,  the  present  science 
of  genetics  has  a  history  largely  confined  to  this  century. 

Although  clearly  intimated  by  a  number  of  workers,  the 
conception  of  the  continuity  of  the  germ  cells  was  first  forced 
upon  the  attention  of  biologists  and  given  greater  precision 
by  WEISMANN  (1834-1914)  of  Germany  in  a  series  of  essays 
culminating  in  1892  in  his  volume  entitled  The  Germ  Plasm. 
He  identified  the  chromatin  material  which  constitutes  the 
chromosomes  of  the  cell  nucleus  as  the  specific  bearer  of 
hereditary  characters,  and  emphasized  a  sharp  distinction 
between  germ  cells  and  somatic  cells. 

While  this  viewpoint  had  been  gradually  gaining  content 
and  precision,  the  science  of  genetics  had  been  advancing  not 
only  by  exact  studies  on  the  structure  and  physiology  of  the 
germ  cells,  but  also  by  statistical  studies  of  the  results  of 
heredity  —  the  various  characters  of  animals  and  plants  as 
exhibited  in  parents  and  offspring.  The  studies  of  this  type 
which  first  attracted  the  attention  of  biologists  were  made  by 
G ALTON  (1822-1911)  of  England.  In  the  eighties  and  nine- 
ties of  the  last  century,  he  amassed  a  great  volume  of  data 
in  regard  to,  for  example,  the  stature  of  children  with  refer- 
ence to  that  of  their  parents,  and  formulated  his  well-known 
'laws'  of  inheritance.  But  the  work  which  eventually 
created  the  science  of  genetics  was  that  of  GREGOR  MENDEL 
(1822-1884)  of  Austria.  Mendel  combined  in  a  masterly 
manner  the  experimental  breeding  of  pedigree  strains  of 
plants  and  the  statistical  treatment  of  the  data  thus  secured 
in  regard  to  the  inheritance  of  sharply  contrasting  characters, 
such  as  the  form  and  color  of  the  seeds  in  Peas.  Mendel's 
work  was  published  in  1863  in  an  obscure  natural  history 
periodical,  and  he  abandoned  teaching  and  research  to  be- 
come the  Abbot  of  his  monastery.  Thus  terminated  pre- 
maturely the  productive  work  of  one  of  the  epochmakers  of 


EPOCHS    TN    BIOLOGICAL   HISTORY  405 

biology,  and  the  now  famous  Mendelian  laws  of  inheritance 
were  unknown  to  science  until  1900,  when  other  biologists, 
coming  to  similar  results,  unearthed  his  forty-year-old  paper. 
We  have  already  seen  that  the  fundamental  principle  of  the 
segregation  of  the  genes  of  'alternative'  characters  in  the 
germ  cells,  which  Mendel's  work  indicated,  has  been  ex- 


FIG.  208.  —  Gregor  Johann  Mendel. 

tended  to  other  plants  and  to  animals,  and  that  instead  of 
being,  as  at  first  thought,  a  principle  of  rather  limited  ap- 
plication, has  come  to  be  the  key  to  all  inheritance.  And 
the  present  results  are  extremely  convincing  because  cyto- 
logical  studies  on  the  architecture  of  the  chromosome  com- 
plex of  the  germ  cells  keep  pace  with  and  afford  a  picture 
of  the  physical  basis  of  inheritance  —  the  mechanism  by 
which  the  segregation  and  distribution  of  characters  by  the 


406  FOUNDATIONS    6F    BIOLOGY 

Mendelian  formula  takes  place.     Such  is  the  deeply  hidden 
modicum  of  truth  in  the  old  preformation  theories! 

7.   Organic  Evolution 

A  question  which  has  interested  and  perplexed  thinking 
men  of  all  times  is  how  things  came  to  be  as  they  are  to-day. 
The  historian  of  human  affairs  attempts  to  trace  the  sequence 
and  relationship  of  events  from  the  remote  past  to  the  pres- 
ent. Similarly,  the  geologist  endeavors  to  formulate  the 
history  of  the  Earth;  and  the  biologist,  the  history  of  plants 
and  animals  on  the  Earth.  All  recognize  that  the  present  is 
the  child  of  the  past  and  the  parent  of  the  future,  and  that 
past,  present,  and  future,  though  causally  related,  are  never 
the  same.  It  was  the  Greek  natural  philosophers  who  pro- 
jected this  idea  of  history  into  science  and  attempted  to 
substitute  a  naturalistic  explanation  of  the  Earth  and  its 
inhabitants  for  the  established  theogonies,  and  thus  started 
the  uniformitarian  trend  of  thought  which  culminated  in  the 
establishment  of  organic  evolution  during  the  past  century. 

Aristotle  held  substantially  the  modern  idea  of  the  evolu- 
tion of  life  from  a  primordial  mass  of  living  matter  to  the 
higher  forms,  and  placed  Man  at  the  head  of  animal  creation. 
"To  him  belongs  the  God-like  nature.  He  is  preeminent  by 
thought  and  volition.  But  although  all  are  dwarf -like  and 
incomplete  in  comparison  with  Man,  he  is  only  the  highest 
point  of  one  continuous  ascent."  And  evolution  is  still  going 
on  —  the  highest  has  not  yet  been  attained.  In  looking  for 
the  effective  cause  of  evolution  Aristotle  rejected  the  hy- 
pothesis of  EMPEDOCLES  (495-435  B.C.),  which  embodied  in 
crude  form  the  idea  of  the  survival  of  the  fittest,  and  substi- 
tuted secondary  natural  laws  to  account  for  the  apparent 
design  in  nature.  This  was  a  sound  induction  by  Aristotle 
from  his  necessarily  limited  knowledge  of  nature,  but  had  he 


EPOCHS    IN    BIOLOGICAL   HISTORY  407 

accepted  the  idea  of  the  survival  of  the  fittest  to  account  for 
adaptations  in  organisms,  he  would  have  been  "the  literal 
prophet  of  Darwinism." 

The  thread  of  continuity  in  evolutionary  thought  is  not 
broken  from  Aristotle  to  the  present,  but  from  the  strictly 
biological  viewpoint  two  Frenchmen,  Buffon  and  Lamarck, 


Fia.  209.  —  Comte  de  Buffon. 

and  two  Englishmen,  Erasmus  Darwin  and  his  grandson, 
Charles  Darwin,  stand  preeminent. 

BUFFON  (1707-1788)  was  a  peculiarly  happy  combination 
of  entertainer  and  scientist  who  found  expression  in  each 
new  volume  of  his  great  Natural  History.  And  it  was  largely, 
so  to  speak,  between  the  lines  of  this  work  that  Buffon's 
evolutionary  ideas  were  displayed;  beyond  the  reach,  he 
hoped,  of  the  censor  and  dilettante.  It  is  not  strange,  there- 
fore, that  it  is  often  difficult  to  decide  just  how  much  weight 
is  to  be  placed  on  some  of  his  statements;  though  certainly 
it  is  not  exaggerating  to  ascribe  to  him  not  only  the  recogni- 


408  FOUNDATIONS   OF   BIOLOGY 

tion  of  the  factors  of  geographical  isolation,  struggle  for 
existence,  artificial  and  natural  selection  in  the  origin  of 
species,  but  also  the  propounding  of  a  theory  of  the  origin 
of  variations  —  that  the  direct  action  of  the  environment 
brings  about  alterations  in  the  structure  of  animals  and 
plants  and  these  are  transmitted  to  the  offspring. 

When  Buffon's  influence  had  passed  its  zenith,  ERASMUS 
DARWIN    (1731-1802)    expressed   consistent   views   on    the 


FIG.  210.  —  Erasmus  Darwin. 

evolution  of  organisms,  in  several  volumes  of  prose  and  poetry, 
which  lead  biologists  to-day  to  recognize  him  as  the  antici- 
pator of  the  Lamarckian  doctrine  that  somatic  variations 
arise  through  the  reaction  of  the  organism  to  environmental 
conditions.  "  All  animals  undergo  transformations  which  are 
in  part  by  their  own  exertions,  in  response  to  pleasures,  and 
pain,  and  many  of  these  acquired  forms  or  propensities  are 
transmitted  to  their  posterity." 


EPOCHS    IN    BIOLOGICAL   HISTORY  409 

LAMARCK  (1744-1829)  developed  with  great  care  the  first 
complete  and  logical  theory  of  organic  evolution  and  is  the 
one  outstanding  figure  in  biological  uniformitarian  thought 
between  Aristotle  and  Charles  Darwin.  "For  nature,"  he 
writes,  "time  is  nothing.  For  all  the  evolution  of  the  Earth 
and  of  living  beings,  nature  needs  but  three  elements,  space, 
time,  and  matter."  In  regard  to  the  factors  of  evolution, 


FIG.  211.  —  Jean-Baptiste  Lamarck. 

Lamarck  put  emphasis  on  the  indirect  action  of  the  environ- 
ment in  the  case  of  animals,  and  the  direct  action  in  the 
case  of  plants.  The  former  are  induced  to  react  and  so 
adapt  themselves,  as  it  were;  while  the  latter,  without  a 
nervous  system,  are  molded  directly  by  their  surroundings. 
And,  so  Lamarck  believed,  such  changes,  somatic  in  origin  — 
acquired  characters  —  are  transmitted  to  the  next  generation 
and  bring  about  the  evolution  of  organisms. 


410  FOUNDATIONS    OF   BIOLOGY 

Through  the  relative  weakness  of  Lamarck's  successors 
the  French  school  of  evolutionists  dwindled  to  practical 
extinction;  while  in  Germany,  GOETHE  (1749-1832),  the 
greatest  poet  of  evolution,  and  TREVIRANUS  (1776-1837) 
"brilliantly  carried  the  argument  without  carrying  convic- 
tion," for  the  man  and  the  moment  must  agree.  Then  in 
England  the  uniformitarian  ideas  of  HUTTON  (1726-1797), 
elaborated  by  LYELL  (1797-1875)  in  his  Principles  of  Geology 
(1830-1833),  established  evolution  in  geology,  and  the  way 
was  paved  for  CHARLES  DARWIN  (1809-1882)  to  do  the  same 
for  the  organic  world.  It  is  true  that  "the  idea  of  develop- 
ment saturated  the  intellectual  atmosphere  —  nevertheless 
the  elaborate  and  toilsome  labor  of  thinking  it  through  for 
the  endless  realm  of  nature  was  to  be  done"  and  Darwin 
did  it  in  his  Origin  of  Species  which  appeared  in  1859.  By 
his  brilliant,  scholarly,  open-minded,  and  cautious  mar- 
shalling of  the  facts  pointing  toward  the  universality  of  varia- 
tions and  the  mutability  of  species;  and  by  the  theory  of 
natural  selection  on  the  basis  of  slight  adaptive  variations 
resulting  in  the  survival  of  the  fittest  in  the  struggle  for 
existence  —  which,  strange  to  say,  Darwin  and  WALLACE 
(1822-1913)  reached  simultaneously  and  independently - 
Darwin  "made  the  old  idea  current  intellectual  coin." 

To-day,  as  we  know,  no  representative  biologist  questions 
the  fact  of  evolution  —  "evolution  knows  only  one  heresy,  the 
denial  of  continuity"  -though  in  regard  to  the  factors 
involved  there  is  much  difference  of  opinion.  It  may  well 
be  that  we  shall  have  reason  to  depart  widely  from  Darwin's 
interpretation  of  the  effective  principles  at  work  in  the  origin 
of  species,  but  withal  this  will  have  little  influence  on  his 
position  in  the  history  of  biology.  The  great  value  which  he 
placed  upon  facts  was  exceeded  only  by  his  demonstration 
that  this  "value  is  due  to  their  power  of  guiding  the  mind  to  a 


EPOCHS    IN    BIOLOGICAL   HISTORY  411 

further  discovery  of  principles."  The  Origin  of  Species 
brought  biology  into  line  with  the  other  inductive  sciences, 
recast  practically  all  of  its  problems,  and  instituted  new  ones. 
Darwin  beautifully  and  conservatively  expressed  this  new 
outlook  on  nature  in  the  historically  important  concluding 
paragraph  of  his  epoch-making  work: 

"It  is  interesting  to  contemplate  a  tangled  bank,  clothed 
with  many  plants  of  many  kinds,  with  birds  singing  on  the 
bushes,  with  various  insects  flitting  about,  and  with  worms 
crawling  through  the  damp  earth,  and  to  reflect  that  these 
elaborately  constructed  forms,  so  different  from  each  other, 
and  dependent  upon  each  other  in  so  complex  a  manner,  have 
all  been  produced. by  laws  acting  around  us.  These  laws, 
taken  in  the  largest  sense,  being  Growth  with  Reproduction; 
Inheritance  which  is  almost  implied  by  reproduction;  Varia- 
bility from  the  indirect  and  direct  action  of  the  conditions  of 
life,  and  from  use  and  disuse:  a  Ratio  of  Increase  so  high  as  to 
lead  to  a  Struggle  for  Life,  and  as  a  consequence  to  Natural 
Selection,  entailing  Divergence  of  Character  and  the  Extinc- 
tion of  less-improved  forms.  Thus,  from  the  war  of  nature, 
from  famine  and  death,  the  most  exalted  object  which  we  are 
capable  of  conceiving,  namely,  the  production  of  the  higher 
animals,  directly  follows.  There  is  a  grandeur  in  this  view 
of  life,  with  its  several  powers,  having  been  originally  breathed 
by  the  Creator  into  a  few  forms  or  into  one;  and  that,  whilst 
this  planet  has  gone  cycling  on  according  to  the  fixed  law 
of  gravity,  from  so  simple  a  beginning  endless  forms  most 
beautiful  and  most  wonderful  have  been,  and  are  being 
evolved." 


APPENDIX 

I.    A  BRIEF  SYNOPTIC  CLASSIFICATION 
OF  PLANTS  AND  ANIMALS 

A.    PLANTS 

Phylum  1.     THALLOPHYTA:    Thallus  plants. 
Series  of  the  ALGAE.     (1500  species.) 

Class      I.  CYANOPHYCEAE  :  Blue-green  Algae.     Oscillatoria. 
Class    II.  CHLOROPHYCEAE  :  Green  Algae. 
Order  1.  Protococcales:    Unicellular  Green  Algae.     Pleuro- 

coccus,  Sphaerella. 
Order  2.  Confervales:  Confervas  and  Sea  Lettuces.  Ulothrix, 

Oedogonium,  Ulva. 
Order  3.  Conjugates:  Pond  Scums,  Desmids,  and  Diatoms. 

Spirogyra,  Closterium,  Navicula. 
Order  4.  Siphonales:  Tubular  Algae.    Vaucheria. 
Order  5.  Charales:  Stoneworts.    Chara. 
Class  III.  PHAEOPHYCEAE  :   Brown  Algae.    Kelps  and  Rock- 
Weeds.    Laminaria,  Fucus,  Sargassum. 
Class  IV.  RHODOPHYCEAE  :  Red  Algae.    Rhodomela. 
Series  of  the  FUNGI.     (65,000  species.) 
Class     V.  SCHIZOMYCETES:  Bacteria. 
Class  VI.  PHYCOMYCETES:  Alga-like  Fungi.     Molds. 
Class  VII.  ASCOMYCETES:  Sac  Fungi.  Mildews,  Morels,  Truffles, 

Yeasts,  (Lichens). 
Class  VEIL  BASIDIOMYCETES:    Basidia  Fungi.     Smuts,  Rusts, 

Toadstools,  Mushrooms. 
Phylum  2.  BRYOPHYTA:     Liverworts    and    Mosses.      (17,000 

species.) 

Class     I.  HEPATICAE:  Liverworts.    Marchantia. 
413 


414  APPENDIX 

Class    II.  Musci:   Mosses. 

Order  1.  Sphagnales:   Peat  Mosses.     Sphagnum. 
Order  2.  Bryales:   Common  Mosses.     Polytrichum,  Bryum. 
Phylum  3.  PTERIDOPH YTA :     Ferns   and   their   allies.      (4500 

species.) 
Class     I.  FILICINEAE:    Common    Ferns    and  Water  Ferns. 

Aspidium,  Marsilia. 

Class    II.  EQUISETINEAE  :  Horsetails.     Equisetum. 
Class  III.  LYCOPODINEAE  :     Lycopods.     Selaginella. 
Phylum  4.  SPERMATOPH YTA :  Seed  Plants.   Flowering  Plants. 
Subdivision    1.   GYMNOSPERMAE:     Cycads    and    Conifers. 

Pines.     (600  species.) 
Subdivision    2.    ANGIOSPERMAE:      The   familiar  'flowering 

plants/ 

Class      I.  MONOCOTYLEDONEAE  :    Grasses,  Palms,  Lilies,   Or- 
chids.    (25,000  species.) 

Class    II.  DICOTYLEDONEAE  :  Elms,  Buttercups,  Pitcher  Plants, 
Roses,  Beans,  Flax,  Cacti,  Daisies.     (110,000  species.) 

B.    ANIMALS 

Phylum  1.  PROTOZOA.     (10,000  species.) 

Class      I.  SARCODINA:   Amoeba,  the  Forarninifera. 

Class    II.  MASTIGOPHORA:  Flagellates.  Euglena,  Volvox,  Try- 
panosoma. 

Class  III.  SPOROZOA:   Plasmodium  malariae. 

Class  IV.  INFUSORIA:  Paramecium,  Vorticella. 
Phylum  2.  PORIFERA:  Sponges.  (2500  species.) 
Phylum  3.  COELENTERATA.  (4500  species.) 

Class      I.  HYDROZOA:  Hydra,  Obelia,  Gonionemus. 

Class    II.  SCYPHOZOA:   Jellyfish. 

Class  III.  ANTHOZOA:   Sea  Anemones,  Corals. 

Class  IV.  CTENOPHORA:   Sea  Combs. 
Phylum  4.    PLATYHELMINTHES:   Flatworms.    (5000  species.) 

Class     I.  TURBELLARIA:   Planaria. 

Class    II.  TREMATODA:   Liver  Flukes, 

Class  III.  CESTODA:  Tape  Worms. 


CLASSIFICATION  415 

Phylum  5.     NEMATHELMINTHES:     Round  Worms.    Ascaris, 
Trichina.     (1500  species.) 

Phylum  6.  TROCHELMINTHES:   Rotifers.     (500  species.) 
Phylum  7.  MOLLUSCOIDA:   Polyzoans  and  Brachiopods.    (2000 
species.) 

Phylum  8.  ECHINODERMATA:    (4000  species.) 

Class      I.  ASTEROIDEA:   Starfishes. 

Class    II.  OPHIUROIDEA:  Serpent  Stars. 

Class  III.  ECHINOIDEA:  Sea  Urchins. 

Class  IV.  HOLOTHUROIDEA:    Sea  Cucumbers. 

Class    V.  CRINOIDEA:   Feather  Stars,  Sea  Lilies. 
Phylum  9.  ANNELIDA.     Segmented  Worms.     (4000  species.) 

Class     I.  ARCHIANNELIDA  :   Polygordius. 

Class    II.  CHAETOPODA:  Earthworms,  Clamworms. 

Class  III.  HIRUDINEA:   Leeches. 
Phylum  10.  MOLLUSCA.     (60,000  species.) 

Class      I.  LAMELLIBRANCHIATA  :     Oysters,    Clams,    Scallops, 
Shipworm. 

Class    II.  AMPHINEURA:   Chiton. 

Class  III.  GASTROPODA:   Snails. 

Class  IV.  SCAPHOPODA:   Dentalium. 

Class     V.  CEPHALOPODA:   Squid,  Octopus,  Nautilus. 
Phylum  11.  ARTHROPOD  A. 

Class      I.  CRUSTACEA:  Barnacles,  Crayfishes,  Lobsters,  Crabs, 
Trilobites  (extinct).     (16,000  species.) 

Class    II.  ONYCHOPHORA:  Peripatus. 

Class  III.  MYRIAPODA:  Centipedes,  Millipedes. 

Class  IV.  INSECTA:  Locusts,  Bugs,  Flies,  Butterflies,  Beetles, 
Ants,  Bees,  Wasps.     (400,000  species.) 

Class    V.  ARACHNIDA:  Scorpions,  Spiders.     (16,000  species.) 
Phylum  12.  CHORDATA. 

Subphylum  A.    ENTEROPNEUSTA:  Dolichoglossus. 
Subphylum    B.     TUN  1C  AT  A:  Tunicates.      Cynthia.       (1500 

species.) 
Subphylum  C.    CEPHALOCHORDA:  Amphioxus. 


416  APPENDIX 

Subphylum  D.     VERTEBRATA. 
Class     I.  CYCLOSTOMATA  :  Lampreys. 
Class    II.  ELASMOBRANCHII:   Sharks.   Dogfish.!  (15,000 
Class  III.  PISCES:  Cod,  Trout,  Perch.  J    species.) 

Class  IV.  AMPHIBIA:  Frogs,    Toads,    Salamanders.       (1400 

species.) 
Class     V.  REPTILIA:     Lizards,    Snakes,    Tortoises,    Turtles, 

Crocodiles,  Dinosaurs  (extinct).    (3500  species.) 
Class  VI.  AVES:   Birds.     (13,000  species.) 
Subclass  1.  Archaeornithes:  Archaeopteryx  (extinct). 
Subclass  2.  Neornithes. 
Division  A.  Ratitae:  Apteryx,  Ostrich. 
Division  B.  Carinatae:  All  familiar  birds. 
Class  VII.  MAMMALIA.     (3500  species.) 

Subclass   1.  Prototheria:  Duck-bill,  Echidna. 
Subclass  2.  Metatheria:  Opossums,  Kangaroos. 
Subclass  3.  Eutheria:    Sloths,   Whales,   Porpoises,    Horses, 
Tapirs,  Camels,  Cats,  Hedgehogs,  Bats,  and  the  Primates 
including  Monkeys,  Apes,  Man. 


II.  BIBLIOGRAPHY 

Some  easily  available  works  in  English  which  are  suitable  for 
reference  and  collateral  reading. 

CHAPTER  I 

COLTON,  H.  S.    A  List  of  Selected  Readings  for  Students  in  Elementary 

College  Zoology.    University  of  Pennsylvania,  1915. 
GREGORY,  R.  A.    Discovery,  or  the  Spirit  of  Service  of  Science.    The 

Macmillan  Co.,  1919. 

HENDERSON,  I.  F.  and  HENDERSON,  W.  D.    A  Dictionary  of  Scien- 
tific Terms:  Pronunciation,  Derivation,  and  Definition  of  Terms 

in  Biology,  Botany,  Zoology,  Anatomy,  Cytology,  Embryology, 

Physiology.     Oliver  &  Boyd,  1920. 
HUXLEY,   T.   H.     "  Educational  Value   of  the   Natural  History 

Sciences."     Collected  Essays,  Vol.  Science  and  Education.     D. 

Appleton  &  Co. 
HUXLEY,  T.  H.      "On   our   Knowledge   of   the   Causes   of   the 

Phenomena  of  Organic  Nature."     Collected  Essays,  Vol.  Dar- 

winiana. 
HUXLEY,  T.  H.     "On  the  Study  of  Biology."     Collected  Essays, 

Vol.  Science  and  Education. 
MILLS,  JOHN.    Realities  of  Modern  Science.     Introduction  for  the 

Modern  Reader.     The  Macmillan  Co.,  1919. 
PEARSON,  KARL.     The  Grammar  of  Science.    3d  edition.    A.  &  C. 

Black,  1911. 
SANFORD,  FERNANDO.     The  Scientific  Method:    Its    History    and 

Its  Value.    The  Macmillan  Co.,  1921. 
THOMSON,  J.  A.    An  Introduction  to  Science.    H.  Holt  &   Co., 

1911. 

WESTAWAY,  F.  W.    Scientific  Method.    Blackie  &  Son,  1912. 

417 


418  APPENDIX 

CHAPTER  II 

BAYLISS,  W.  M.     Principles  of  General  Physiology.    3d  Edition. 

Longmans,  Green  &  Co.,  1921. 
EULER,  HANS.     General  Chemistry  of  the  Enzymes.    John  Wiley  & 

Sons,  1912. 
HARROW,  BENJAMIN.     Vitamines:    Essential  Food  Factors.    E.  P. 

Button  &  Co.,  1921. 
HUXLEY,  T.  H.     "On  the  Physical  Basis  of  Life."    Collected  Essays, 

Vol.  Method  and  Results.     D.  Appleton  &  Co. 
LOEB,    JACQUES.     The    Dynamics    of   Living    Matter.    Columbia 

University  Press,  1906. 
SHERMAN,  H.  C.     Chemistry  of  Food  and  Nutrition.    2d  Edition. 

The  Macmillan  Co.,  1918. 

SLOSSON,  E.  E.    Creative  Chemistry.    The  Century  Co.,  1920. 
TAYLOR,  W.  W.     The  Chemistry  of  Colloids  and  some  Technical 

Applications.     Longmans,  Green  &  Co.,  1915. 
UNDERBILL,  F.  P.    Physiology  of  the  Amino  Acids.    Yale  Univer- 
sity Press,  1915. 

CHAPTER   III 

AGAR,  W.  E.  Cytology,  with  Special  Reference  to  the  Metazoan 
Nucleus.  The  Macmillan  Co.,  1920. 

DONCASTER,  L.  An  Introduction  to  the  Study  of  Cytology.  Cam- 
bridge University  Press,  1920. 

SHARP,  L.  W.  Introduction  to  Cytology.  McGraw-Hill  Book  Co., 
1921. 

THOMPSON,  D'ARCY  W.  On  Growth  and  Form.  Cambridge 
University  Press,  1917. 

WILSON,  E.  B.  The  Cell  in  Development  and  Inheritance.  Columbia 
University  Press,  1900. 

CHAPTER  IV 
DENDY,  ARTHUR.    Outlines  of  Evolutionary  Biology.     D.  Appleton 

&  Co.,  1911. 
DUGGAR,  B.  M.     Plant  Physiology,  with  Special  Reference  to  Plant 

Production.     The  Macmillan  Co.,  1911. 


BIBLIOGRAPHY  419 

GANONG,    W.    F.     Textbook   of  Botany  for   Colleges.    The   Mac- 

millan  Co.,  1917. 
PEEBLES,  FLORENCE.  "Life  History  of  Sphaerella  lacustris."  Central- 

blattfilr  Bakteriologie,  1909. 
THATCHER,  R.  W.     The  Chemistry  of  Plant  Life.     McGraw-Hill 

Book  Co.,  1921. 

CHAPTER  V 

CALKINS,  G.  N.    Protozoology.    Lea  and  Febiger,  1909. 
HUXLEY,  T.  H.     "On  the  Border  Territory  between  the  Animal 

and  Vegetable  Kingdoms."     Collected  Essays,  Vol.     Discourses 

Biological  and  Geological. 
MINCHIN,  E.  A.  Introduction  to  the  Study  of  the  Protozoa.  Arnold, 

1912. 
SEDGWICK,  W.  T.  and  WILSON,  E.  B.    General  Biology.    Henry 

Holt  &  Co.,  1895. 

CHAPTER  VI 

BUCHANAN,  E.  U.  and  BUCHANAN,  R.  E.  Bacteriology.  Revised 
Edition.  The  Macmillan  Co.,  1921. 

FROST,  W.  D.  and  MCCAMPBELL,  E.  F.  Textbook  of  General  Bac- 
teriology. The  Macmillan  Co.,  1910. 

MUIR,  ROBERT  and  RITCHIE,  JAMES.  Manual  of  Bacteriology. 
4th  Edition.  The  Macmillan  Co.,  1907. 

CHAPTER  VII 

BOHM,  A.  A.  and  VON  DAVIDOFF,  M.  A  Textbook  of  Histology, 
Including  Microscopic  Technic.  Edited  by  G.  Carl  Huber. 
2d  Edition.  W.  B.  Saunders  Co.  1914. 

CHAMBERLAIN,  C.  J.  Methods  in  Plant  Histology.  3d  edition.  Uni- 
versity of  Chicago  Press,  1915, 

DAHLGREN,  ULRIC  and  KEPNER,  W.  A.  Principles  of  Animal  Histol- 
ogy. The  Macmillan  Co.,  1908. 

GUYER,  M.  F.  Animal  Micrology.  Practical  Exercises  in  Zo- 
ological Micro-technique.  2d  edition.  University  of  Chicago 
Press,  1917. 

KELLICOTT,  W.  E.     General  Embryology.     H.  Holt  &  Co.,  1913. 


420  APPENDIX 

STEVENS,  W.  C.  Plant  Anatomy  from  the  Standpoint  of  the  Develop- 
ment and  Functions  of  the  Tissues.  •  P.  Blakiston's  Sons  &  Co., 
1911. 

CHAPTERS  VIII  and  IX 

BERGEN,  J.  Y.  and  CALDWELL,  0.  W.    Practical  Botany.    Ginn  & 

Co.,  1911. 
BERGEN,  J.  Y.  and  DAVIS,  B.  M.     Principles  of  Botany.     Ginn  & 

Co.,  1906. 
CAMPBELL,  D.  H.    A  University  Textbook  of  Botany.    The  Mac- 

millan  Co.,  1907. 
COULTER,  J.  G.    Plant  Life  and  Plant  Uses.    American  Book  Co., 

1913. 
COULTER,  J.  M.     The  Evolution  of  Sex  in  Plants.    University  of 

Chicago  Press,  1914. 
COULTER,  J.  M.,  BARNES,  C.  R.  and  COWLES,  H.  C.      Textbook 

of  Botany.    American  Book  Co.,  1910. 
DENSMORE,  H.  D.    General  Botany.     Ginn  &  Co.,  1920. 
GAGER,  C.  S.    Fundamentals  of  Botany.     P.  Blakiston's  Son  &  Co., 

1916. 
GANONG,  W.  F.      Textbook  of  Botany  for  Colleges.     The  Mac- 

millan  Co.,  1917. 
GRAY,  ASA.     Manual  of  Botany.    7th  edition.    American  Book 

Co.,  1908. 
STRASBURGER'S  Textbook  of  Botany.    5th  English  Edition.     The 

Macmillan  Co.,  1921. 

CHAPTERS  X-XV 

BEDDARD*  F.  E.  Earthworms  and  their  Allies.  Cambridge  Uni- 
versity Press,  1901. 

Cambridge  Natural  History.  Ten  volumes.  S.  F.  Harmer  and 
A.  E.  Shipley,  Editors.  The  Macmillan  Co.,  1895. 

CONN,  H.  W.  and  BUDINGTON,  R.  A.  Physiology  and  Hygiene. 
Silver,  Burdett  &  Co.,  1909. 

DREW,  G.  A.  Invertebrate  Zoology.  3d  edition,  revised.  W.  B. 
Saunders  Co.,  1920. 


BIBLIOGRAPHY  421 

HEGNER,  R.  W.    Introduction  to  Zoology.    The  Macmillan  Co., 

1913. 

HEGNER,  R.  W.     College  Zoology.    The  Macmillan  Co.,  1914. 
HOLMES,  S.  J.    Biology  of  the  Frog.    The  Macmillan  Co.,  1914. 
HOUGH,  T.  and  SEDGWICK,  W.  T.     The  Human  Mechanism.    Re- 
vised edition.     Ginn  &  Co.,  1918. 
HOWELL,  W.  H.     Textbook  of  Physiology.    W.  B.  Saunders  Co., 

7th  edition.     1920. 

HUXLEY,  T.  H.     The  Crayfish.     1880. 
HUXLEY,  T.  H.     Lessons  in  Elementary  Physiology.     6th   edition. 

The  Macmillan  Co.,  1915. 
HYMAN,  L.  H.     A  Laboratory  Manual  for  Comparative  Vertebrate 

Anatomy.     University  of  Chicago  Press,  1922. 
KEITH,  ARTHUR.     The  Engine  of  the  Human  Body.    J.  B.  Lippin- 

cott  Co.,  1920. 
KINGSLEY,    J.    S.     Comparative    Anatomy.    P.    Blakiston's  Sons 

&  Co.,  1912. 

KINGSLEY,  J.  S.     Vertebrate  Zoology.    H.  Holt  &  Co.,  1899. 
LANKESTER,  E.  R.  (editor),  Treatise  on  Zoology.     Eight  volumes. 

The  Macmillan  Co. 
LILLIE,  F.  R.     "The  Free-Martin;  A  Study  of  the  Action  of  Sex 

Hormones  in  the  Foetal  Life  of  Cattle."   Jour.  Exp.  Zool.,  Vol. 

23,  1917. 
LINVILLE,  H.  R.  and  KELLY,  H.  A.     Textbook  in  General  Zoology. 

Ginn  &  Co.,  1906. 
MARSHALL,  F.  H.  A.     The  Physiology  of  Reproduction.    Longmans, 

Green  &  Co.,  1910. 
MARTIN,  H.  N.     The  Human  Body.     10th  edition.     H.  Holt  & 

Co.,  1917. 

NEWMAN,  H.  H.     Vertebrate  Zoology.    The  Macmillan  Co.,  1920. 
OSBORN,  HERBERT.    Economic  Zoology.    An  Introductory  Text- 
book in  Zoology.     The  Macmillan  Co.,  1912. 
PARKER,  G.  H.     The  Elementary  Nervous  System.    J.  B.  Lippincott 

Co.,  1919. 
PARKER,  T.  J.  and  HASWELL,  W.  A.     Textbook  of  Zoology.    3d 

edition.     The  Macmillan  Co.,  1922. 


422  APPENDIX 

PETKUNKEVITCH,  ALEXANDER.  Morphology  of  Invertebrate  Types. 
The  Macmillan  Co.,  1916. 

PRATT,  H.  S.  A  Manual  of  the  Common  Invertebrate  Animals, 
Exclusive  of  Insects.  A.  C.  McClurg  &  Co.,  1916. 

REYNOLDS,  S.  H.  The  Vertebrate  Skeleton.  2d  edition.  Cam- 
bridge University  Press,  1913. 

SHIPLEY,  A.  E.  and  MACBRIDE,  E.  W.  Zoology.  The  Macmillan 
Co.,  1901. 

WALTER,  H.  E.     The  Human  Skeleton.    The  Macmillan  Co.,  1918. 

WARD,  H.  B.  and  WHIPPLE,  G.  C.  Fresh-Water  Biology.  John 
Wiley  &  Sons,  1918. 

WILDER,  H.  H.     History  of  the  Human  Body.     H.  Holt  &  Co.,  1909. 

CHAPTER  XVI 

CALKINS,  G.  N.     Biology.     H.  Holt  &  Co.,  1917. 

CHILD,  C.  M.  Senescence  and  Rejuvenescence.  University  of 
Chicago  Press,  1915. 

CONKLIN,  E.  G.  Localization  of  Morphogenetic  Substances  in  the 
Egg.  J.  B.  Lippincott  Co.,  1922. 

DRIESCH,  HANS.  Science  and  Philosophy  of  the  Organism.  Gifford 
Lectures,  1907-08.  A.  &  C.  Black. 

GEDDES,  P.  and  THOMSON,  J.  A.    Sex.    H.  Holt  &  Co.,  1914. 

HEGNER,  R.  W.  The  Germ-cell  Cycle  in  Animals.  The  Mac- 
millan Co.,  1914. 

HUXLEY,  T.  H.  "  Biogenesis  and  Abiogenesis."  Collected  Essays, 
Vol.  Discourses  Biological  and  Geological. 

JENNINGS,  H.  S.  Life  and  Death,  Heredity  and  Evolution  in  Uni- 
cellular Organisms.  Gorham  Press,  1920. 

KELLICOTT,  W.  E.    Chordate  Development.    H.  Holt  &  Co.,  1913. 

LILLIE,  F.  R.  Problems  of  Fertilization.  University  of  Chicago 
Press,  1919. 

MORGAN,  T.  H.    Regeneration.    Columbia  University  Press,  1901. 

WEISMANN,  AUGUST.  The  Germ  Plasm.  Chas.  Scribner's  Sons,  1892. 

WILSON,  E.  B.     "The  Problem  of  Development,"    Science,  1905. 

WOODRUFF,  L.  L.     "The  Origin  of  Life,"  in  the  Evolution  of  the 


BIBLIOGRAPHY  423 

Earth  and  its  Inhabitants,  R.  S.  Lull,  editor.    3d  edition.    Yale 
University  Press,  1922. 

CHAPTER  XVII 

BABCOCK,  E.  B.  and  CLAUSEN,  R.  E.    Genetics  in  Relation  to  Agri- 
culture.   McGraw-Hill  Book  Co.,  1918. 

BATESON,  WILLIAM.     Materials  for  the  Study  of  Variation.     The 
Macmillan  Co.,  1894. 

BATESON,     WILLIAM.    Problems     of    Genetics.    Yale     University 
Press,  1913. 

CASTLE,  W.  E.    Genetics  and  Eugenics.    Revised  edition.    Har- 
vard University  Press,  1921. 

CONKLIN,  E.  G.     Heredity  and  Environment  in  the  Development  of 
Men.     4th  edition.     Princeton  University  Press,  1922. 

COULTER,  J.  M.  and  COULTER,  M.  C.     Plant  Genetics.    University 
of  Chicago  Press,  1918. 

CUNNINGHAM,   J.   T.     Hormones  and  Heredity.    The   Macmillan 
Company,  1922. 

DAVENPORT,  C.  B.    Heredity  in  Relation  to  Eugenics.    H.  Holt 
&  Co.,  1911. 

EAST,  E.  M.  and  JONES,  D.  F.     Inbreeding  and  Outbreeding;  their 
Genetic  and  Sociological  Significance.     J.  B.  Lippincott  Co.,  1919. 

GALTON,  FRANCIS.    Natural  Inheritance.     1889. 

GODDARD,  H.  H.     The  Kallikak  Family.    A  Study  in  the  Heredity 
of  Feeble-mindedness.     The  Macmillan  Co.,  1912. 

GUYER,  M.  F.     Being  Well-born.    Bobbs  Merrill  Co.,  1916. 

JENNINGS,  H.  S.     "Heredity  and  Personality."    Science,  1911. 

KELLICOTT,   W.   E.     The   Social  Direction   of  Human  Evolution. 
D.  Appleton  &  Co.,  1911. 

MORGAN,  T.  H.   Heredity  and  Sex.  Columbia  University  Press,  1913. 

MORGAN,  T.  H.     The  Physical  Basis  of  Heredity.    J.  B.  Lippin- 
cott &  Co.,  1919. 

MOTT,  F.  W.    Nature  and  Nurture  in  Mental  Development.    London, 
1914. 

PEARL,   RAYMOND.    Modes  of  Research  in  Genetics.    The  Mac- 
millan Co.,  1915. 


424  APPENDIX 

POPENOE,  P.  and  JOHNSON,  R.  H.     Applied  Eugenics.     The  Mac- 

millan  Co.,  1918. 

PUNNETT,  R.  C.  Mendelism.  6th  edition.  The  Macmillan  Co.,  1919. 
THOMSON,  J.  A.  Heredity.  2d  edition.  Henry  Holt  &  Co.,  1916. 
WALTER,  H.  E.  Genetics.  2d  edition.  The  Macmillan  Co.,  1922. 

CHAPTER  XVIII 

ADAMS,  C.  C.    A  Guide  to  the  Study  of  Animal  Ecology.    The 

Macmillan  Co.,  1913. 
CAMERON,  E.  H.     Psychology  and  the  School.    The  Century  Co., 

1921. 
CHANDLER,  A.  C.    Animal  Parasites  and  Human  Disease.    John 

Wiley  &  Sons,  1918. 

CHESHIRE,  F.  R.     Bees  and  Bee-Keeping.     London,  1886. 
CRILE,  G.  W.     Man — An  Adaptive  Mechanism.    The  Macmillan 

Co.,  1916. 
DARWIN,   CHARLES.     The  Fertilization  of  Orchids.    The  Various 

Contrivances  by  which  Orchids  are  Fertilized  by  Insects.    1862. 
HALDANE,   J.   S.    Organism  and  Environment.    Yale   University 

Press,  1917. 
HENDERSON,  L.  J.     The  Fitness  of  the  Environment.    The  Macmillan 

Co.,  1913. 
HENDERSON,   L.  J.     The  Order  of  Nature.     Harvard  University 

Press,  1917. 
HOLMES,  S.  J.     The  Evolution  of  Animal  Intelligence.    H.  Holt 

&  Co.,  1911. 
JENNINGS,   H.   S.     Behavior  of  the  Lower  Organisms.     Columbia 

University  Press,  1906. 
LLOYD,  R.  E.     What  is  Adaptation?    Longmans,  Green  and  Co., 

1914. 

LOEB,  JACQUES.     The  Organism  as  a  Whole.     New  York,  1916. 
LOEB,  JACQUES.     Forced  Movements,  Tropisms,  and  Animal  Conduct. 

J.  B.  Lippincott,  1918. 
LONGLEY,  W.  H.     "Studies  upon  the  Biological  Significance  of 

Animal  Coloration."    I.  Journ.  of  Exper.  Zoology,  Vol.  23,  1917. 

II.  American  Naturalist,  Vol.  51,  1917. 


BIBLIOGRAPHY  425 

MACE,  H.    A  Book  about  the  Bee.     E.  P.  Button  &  Co.,  1921. 

MORGAN,  T.  H.  Evolution  and  Adaptation.  The  Macmillan  Co., 
1903. 

NEEDHAM,  J.  G.  and  LLOYD,  J.  T.  Life  of  Inland  Waters.  Corn- 
stock  Publishing  Co.,  1916. 

ROOSEVELT,  THEODORE.  "Revealing  and  Concealing  Coloration 
in  the  Birds  and  Mammals."  Bull.  Amer.  Museum  Nat.  Hist., 
XXX,  1911. 

SUMNER,  F.  B.  "Adaptation  and  the  Problem  of  'Organic  Purpose- 
fulness.'  "  American  Naturalist,  1919. 

Symposium,  on  Adaptation.  Papers  by  M.  M.  Metcalf,  B.  E. 
Livingston,  G.  H.  Parker,  A.  P.  Mathews  and  L.  J.  Henderson. 
American  Naturalist,  Vol.  47,  1913. 

THAYER,  G.  H.  Concealing  Coloration  in  the  Animal  Kingdom. 
The  Macmillan  Co.,  1909. 

THOMSON,  J.  A.  The  Study  of  Animal  Life.  4th  edition.  John 
Murray,  1917. 

THOMSON,  J.  A.  The  System  of  Animate  Nature.  H.  Holt  &  Co., 
1920. 

VAN  BENEDEN,  P.  J.  Animal  Parasites  and  Messmates.  D. 
Appleton  &  Co.,  1876. 

WASHBURN,  M.  F.  The  Animal  Mind.  A  Textbook  of  Compara- 
tive Psychology.  The  Macmillan  Co.,  2d  edition,  1917. 

ZINSSER,  HANS.  Injection  and  Resistance,  2d  edition.  The  Mac- 
millan Co.,  1918. 

CHAPTER  XIX 

ALLEN,   J.   A.     "The  Geographical  Distribution  of  Mammals." 

Bulletin  U.  S,  Geological  Survey,  1878. 
BARRELL,  JOSEPH.     "The  Origin  of  the  Earth,"  in  the  Evolution  of 

the  Earth  and  its  Inhabitants.     R.  S.  Lull,  editor.     3d  edition. 

Yale  University  Press,  1922. 

BERGSON,  HENRI.     Creative  Evolution.    English  translation,  1911. 
CAMPBELL,  D.  H.    Plant  Life  and  Evolution.    H.  Holt  &  Co.,  1911. 
CONKLIN,  E.  G.    Direction  of  Human  Evolution.    Chas.  Scribner's 

Sons,  1921. 


426  APPENDIX 

CRAMPTON,  H.  E.  The  Doctrine  of  Evolution,  its  Basis  and  its  Scope. 
Columbia  University  Press,  1911. 

DARWIN,  CHARLES.  Voyage  of  the  Beagle.  (A  Naturalist's  Voy- 
age.) London,  1839. 

DARWIN,  CHARLES.  The  Origin  of  Species.  London,  1859.  6th 
edition,  1880. 

DARWIN,  CHARLES.     The  Descent  of  Man.    London,  1871. 

DARWIN,  CHARLES.  Variation  in  Animals  and  Plants  under  Domes- 
tication. London,  1868. 

Fifty  Years  of  Darwinism:  Modern  Aspects  of  Evolution.  Cen- 
tennial Addresses  in  honor  of  Charles  Darwin  before  the  Ameri- 
can Association  for  the  Advancement  of  Science,  1909. 

GADOW,  HANS.  The  Wanderings  of  Animals.  Cambridge  Univer- 
sity Press,  1913. 

GEDDES,  P.  and  THOMSON,  J.  A.     Evolution.    H.  Holt  &  Co.,  1911. 

HARDY,  M.  E.  An  Introduction  to  Plant  Geography.  Oxford 
University  Press,  1913. 

HOLMES,  S.  J.  The  Trend  of  the  Race.  Harcourt,  Brace  &  Co., 
1921. 

JOHNSTONE,  JAMES.  The  Philosophy  of  Biology.  Cambridge 
University  Press,  1914. 

JORDAN,  D.  S.  and  KELLOGG,  V.  L.  Evolution  and  Animal  Life.  D. 
Appleton  &  Co.,  1907. 

KELLOGG,  V.  L.    Darwinism  To-day.     H.  Holt  &  Co.,  1907. 

LULL,  R.  S.    Organic  Evolution.    The  Macmillan  Co.,  1917. 

NEWMAN,  H.  H.  Readings  in  Evolution,  Genetics,  and  Eugenics. 
University  of  Chicago  Press,  1921. 

NUTTALL,  G.  H.  F.  Blood  Immunity  and  Blood  Relationships. 
Cambridge  University  Press,  1904. 

OSBORN,  H.  F.  The  Origin  and  Evolution  of  Life.  Chas.  Scribner's 
Sons,  1917. 

REICHERT,  E.  T.  and  BROWN,  A.  P.  "The  Differentiation  and 
Specificity  of  Corresponding  Proteins  and  Other  Vital  Sub- 
stances in  Relation  to  Biological  Classification  and  Organic 
Evolution.  The  Crystallography  of  Hemoglobins."  Carnegie 
Institution  of  Washington,  Publication  116,  1909. 


BIBLIOGRAPHY  427 

SCHUCHERT,  CHARLES.  "The  Earth's  Changing  Surface  and  Cli- 
mate," in  the  Evolution  of  the  Earth  and  its  Inhabitants,  R.  S. 
Lull,  editor.  3d  edition.  Yale  University  Press,  1922. 

SCOTT,  W.  D.     The  Theory  of  Evolution.    The  Macmillan  Co.,  1911. 

DEVRIES,  HUGO.  Species  and  Varieties.  Their  Origin  by  Muta- 
tion. 3d  edition.  Open  Court  Publishing  Co.,  1912. 

WALLACE,  A.  R.  Darwinism.  3d  edition,  The  Macmillan  Co., 
1905. 

WALLACE,  A.  R.  The  Geographical  Distribution  of  Animals.  Lon- 
don, 1876. 

WALLACE,  A.  R.  Island  Life.  2d  edition,  The  Macmillan  Co., 
1892. 

WELLS,  H.  G.  The  Outline  of  History,  Chapters  I-XII.  The  Mac- 
millan Co.,  1920. 

Yale  Sigma  Xi  Lectures:  Evolution  of  the  Earth  and  its  Inhabitants, 
3d  edition,  1922;  Evolution  of  Man,  1922.  Yale  University 
Press. 

CHAPTER  XX 

BUTLER,  SAMUEL.    Evolution  Old  and  New.     Revised  edition,  E.  P. 

Button  &  Co.,  1911. 
FOSTER,  MICHAEL.     History  of  Physiology  during  the  16th,  17th,  and 

18th  Centuries.     Cambridge  University  Press,  1901. 
GARRISON,    F.    H.     History    of    Medicine.     3d    edition.    W.    B. 

Saunders  Co.,  1921. 
GREEN,  J.  R.     History  of  Botany,  1860-1900.     Oxford  University 

Press,  1909. 
HUXLEY,  T.  H.     "The  Progress  of  Science,  1837-1887."     Collected 

Essays,  Vol.  Methods  and  Results.     D.  Appleton  &  Co. 
JUDD,  J.  W.     The  Coming  of  Evolution.     The  Story  of  a  Great  Rev- 
olution in  Science.     Cambridge  University  Press,  1910. 
LOCY,  W.  A.    Biology  and  Its  Makers.   3d  edition.    H.  Holt  &  Co., 

1915. 
MERZ,  J.  T.     History  of  Scientific  Thought  in  the  Nineteenth  Century. 

W.  Blackwood  &  Sons,  1903-1914. 
MIALL,  L.  C.     History  of  Biology.     G.  P.  Putnam's  Sons,  1911. 


428  APPENDIX 

OSBORN,  H.  F.  From  the  Greeks  to  Darwin.  An  Outline  of  the 
Development  of  the  Evolution  Idea.  Columbia  University 
Press,  1894. 

VON  SACHS,  JULIUS.  History  of  Botany,  1530-1860.  (English 
translation.)  Oxford  University  Press,  1890. 

THOMPSON,  D'ARCY  W.  On  Aristotle  as  a  Biologist.  Oxford  Uni- 
versity Press,  1913. 

THOMSON,  J.  A.  The  Science  of  Life.  An  Outline  of  the  History  of 
Biology.  Blackie  &  Son,  Ltd.,  1900. 

WHITE,  A.  D.  A  History  of  the  Warfare  of  Science  with  Theology. 
D.  Appleton  &  Co.,  1896. 

WOODWARD,  H.  B.  History  of  Geology.  G.  P.  Putnam's  Sons, 
1911. 

Yale  Gamma  Alpha  Lectures:  History  of  the  Sciences.  Yale  Univer- 
sity Press,  1922. 


III.    GLOSSARY 

ABIOGENESIS.  The  abandoned  idea  that  living  matter  may  arise 
from  non-living  without  the  influence  of  the  former.  See  Bio- 
genesis. 

ABSORPTION.  The  passage  of  nutritive  and  other  fluids  into  living 
cells. 

ACOELOMATE.  Not  possessing  a  coelom,  or  body  cavity.  E.g.,  Hydra. 

ACQUIRED  CHARACTER.  A  modification  of  body  structure  or  func- 
tion which  arises  during  individual  life  as  a  result  of  environ- 
mental influences. 

ADAPTATION.  The  reciprocal  fitness  of  organism  and  environment; 
a  structure  or  reaction  fitted  for  a  special  environment;  the 
process  by  which  an  organism  becomes  fitted  to  its  surroundings. 

ADRENALS.  Suprarenal  bodies.  Ductless  glands  situated  near 
the  kidneys.  Secretion  supplies  a  hormone  known  as  adrenin. 

ADVENTITIOUS.     Not  in  the  usual  position,  e.g.,  aerial  roots. 

AEROBE.     An  organism  requiring  free  oxygen.     See  Anaerobe. 

AFFERENT  ROOT.  Dorsal,  or  posterior,  root  of  certain  cranial  and 
all  spinal  nerves  through  which  sensory  nerve  impulses  enter  the 
brain  and  spinal  cord.  See  Efferent  Root. 

ALGAE.  A  heterogeneous  group  of  lower  plants  in  which  the  body 
is  unicellular  or  consists  of  a  thallus;  e.g.,  Sphaerella,  Spirogyra, 
Seaweeds. 

ALIMENTARY  CANAL.     The  digestive  tract. 

ALLELOMORPHS.  Genes  similarly  situated  on  homologous  chromo- 
somes which  produce  'alternative/  or  'contrasting/  characters. 

ALTERNATIVE  INHERITANCE.     Typical  Mendelian  inheritance. 

AMINO  ACID.  Components  of  proteins.  Organic  acids  in  which  one 
hydrogen  atom  is  replaced  by  the  amino  group  (NH2).  Mono- 
amino  acids,  e.g.,  Glycine  (CH2NH2.COOH).  Diamino  acids, 
e.g.,  Lysine  (H2NCH2.CH2.CH2.CH2.CHNH2.COOH). 

429 


430  APPENDIX 

AMOEBOID.    Usually  applied  to  the  flowing  movements  of  a  cell,  as 

in  the  Protozoon,  Amoeba. 
AMPHIMIXIS.    The  mingling  of  the  germ  plasm  of  two  gametes  in  the 

zygote. 
ANABOLISM.    The  constructive  phase  of  metabolism.    See  Katabo- 

lism. 
ANAEROBE.    An  organism  not  requiring  free  oxygen;  e.g.,  certain 

Bacteria  and  parasitic  Worms.    See  Aerobe. 
ANALOGY.     Structural  resemblance  due  to  similarity  of  function. 

See  Homology. 

ANAPHASE.     Period  in  mitosis  during  which  the  daughter  chromo- 
somes move  toward  the  respective  centrosomes.    See  Telophase. 
ANATOMY.    The  structure  of  organisms,  especially  as  revealed  by 

dissection. 
ANTHER.    The  part  of  the  stamen  which  contains  the  pollen  sacs 

(microsporangia)  in  Flowering  Plants. 
ANTHERIDIUM.    The  organ  in  plants,  such  as  the  Mosses  and  Ferns, 

in  which  the  male  gametes  arise. 
ANUS.     Terminal  orifice  of  the  alimentary  canal.    Opening  of  the 

large  intestine  either  on  the  surface  of  the  body  (Man)  or  into 

the  cloaca  (Frog). 
AORTA.    A  great  trunk  artery  carrying  blood  away  from  the  heart. 

See  Dorsal  Aorta. 
AORTIC   ARCHES.    Arteries   arising  from   the   ventral   aorta   and 

supplying  the  gills  in  aquatic  Vertebrates.      Undergo  many 

modifications  in  the  ascending  series  of  air-breathing  Vertebrates. 
APHIDS.     Small  sucking  Insects;  e.g.,  the  green  'Plant  Lice'  of 

garden  shrubs. 
ARCHEGONIUM.    The  organ  in  plants,  such  as  the  Mosses  and  Ferns, 

in  which  the  female  gamete  (egg)  arises. 
ARTHROPODA.     Phylum  of  Invertebrates.    Includes  the  Crustacea, 

Insecta,  Arachnida,  etc. 

ASTER.    Radiations  surrounding  the  centrosome  during  cell  divi- 
sion. 
ATAVISM.    Appearance  of  grandparental  characters  in  an  individual. 

See  Reversion. 


GLOSSARY  431 

AUTONOMIC  SYSTEM.  System  of  outlying  ganglia  and  nerves  which 
communicates  with  the  central  nervous  system  via  the  roots  of 
the  spinal  and  cranial  nerves.  Innervates  chiefly  the  involuntary 
muscles  of  blood  vessels,  digestive  organs,  etc.  Sympathetic 
system. 

AXON.  A  nerve  fiber  conducting  impulses  away  from  the  cell  body. 
Dendrites  conduct  toward  the  cell  body.  See  Neuron. 

BAST.     The  phloem  portion  of  a  vascular  bundle. 

BIENNIAL.  A  plant  which  completes  its  life  history  in  two  years, 
usually  reproducing  in  the  second. 

BILE  DUCT.  Tube  which  conveys  the  secretions  (bile)  of  the  liver 
to  the  small  intestine.  Usually  unites  with  the  pancreatic  duct 
to  form  a  common  duct  which  enters  the  intestine. 

BINARY  FISSION.  The  division  of  a  cell,  especially  a  unicellular 
organism,  into  two  daughter  cells;  e.g.,  in  Paramecium. 

BINOMIAL  NOMENCLATURE.  The  accepted  scientific  method  of 
designating  organisms  by  two  Latin  or  Latinized  words,  the 
first  indicating  the  genus  and  the  other,  the  species.  E.g.,  the 
Dog,  Canis  familiaris;  Man,  Homo  sapiens. 

BIOGENESIS.  The  established  doctrine  that  all  life  arises  from  pre- 
existing living  matter.  See  Abiogenesis. 

BIOLOGY.  The  study  of  the  manifestations  of  matter  in  the  living 
state. 

BIPARENTAL.  Derived  from  two  progenitors,  male  and  female,  e.g., 
in  sexual  reproduction.  See  Uniparental. 

BLASTOCOEL.   The  cavity  within  the  blastula.  Segmentation  cavity. 

BLASTOPORE.  The  opening  to  the  exterior  from  the  enteric  pouch 
of  a  gastrula. 

BLASTULA.  The  stage  following  cleavage  when  the  cells  are  ar- 
ranged in  a  single  layer  to  form  a  hollow  sphere. 

BLENDING  INHERITANCE.  Apparent  fusion  of  parental  characters 
in  the  offspring  so  that  a  more  or  less  intermediate  condition 
arises.  E.g.,  skin  color  of  mulattoes. 

BLOOD  CORPUSCLES.  Detached  cells  present  in  the  fluid  plasma  of 
the  blood.  Two  principal  kinds,  red  and  white. 

BUCCAL  CAVITY.     Mouth  cavity. 


432  APPENDIX 

BUD.     Growing  point  of  shoot.    An  undeveloped  branch.     Leaf 

buds  form  stem  and  leaves;  mixed  buds,  both  leaves  and  flowers; 

flower  buds,  flowers  only. 
CALCIFEROUS  GLANDS.    Glands  opening  into  the  oesophagus  of  the 

Earthworm  which  secrete  calcium  carbonate,  probably  to  neu- 
tralize acidity  of  food. 
CALYX.    The  outer  whorl  of  modified  leaves  composing  a  typical 

flower.     Usually  green. 
CAMBIUM.    Layer  of  actively  dividing  cells  which,  in  the  highest 

Flowering  Plants,  is  situated  between  xylem  and  phloem  of  vas- 
cular bundles,  and  forms  a  thin  cylinder  between  wood  and  bark. 
CARBOHYDRATES.    Compounds  of  carbon  with  hydrogen  and  oxygen^ 

the  hydrogen  and  oxygen  being  in  the  same  proportion  as  in 

water  (H20). 
CARPEL.     One  of  the  innermost  whorl  of  floral  leaves  which  bear 

the  megaspores.    A  simple  pistil  or  an  element  of  a  compound 

pistil.    A  megasporophyll. 
CATALYSIS.    The  acceleration  of  a  chemical  reaction  by  a  substance 

which  itself  remains  unchanged  (e.g.,  an  enzyme). 
CELL.    A  structural  and  physiological  unit  mass  of  protoplasm, 

differentiated  into  cytoplasm  and  nucleus. 
CELL  SAP.    Water,  with  solutes,  under  pressure  in  a  large  vac- 

uole  in  the  cytoplasm  of  certain  types  of  plant  cells.     Effects 

cell  turgor. 
CELLULOSE.    A  carbohydrate  which  characteristically  forms  the 

walls  of  plant  cells. 
CENTROSOME.     A  minute  body  situated  in  the  center  of  the  aster 

and  active  during  cell  division. 
CHELIPED.     The  first  thoracic  appendages,  or  walking-legs,  in  the 

Crayfish  and  its  allies.    The  'pincer/ 
CHEMOSYNTHESIS.     Manufacture  (synthesis)  of  food  material  from 

water  and  carbon  dioxide,  through  energy  derived  from  chemical 

changes  involving  oxidation  instead  of  directly  from  sunlight. 

Restricted  to  special  groups  of  Bacteria. 
CHEMOTAXIS.     Movements  of  cells  (e.g.,  Paramecium)  in  response 

to  chemical  stimuli. 


GLOSSARY  433 

CHLORENCHYMA.     The  chlorophyll-bearing  tissue  of  plants. 
CHLOROPHYLL.    The  characteristic  green  coloring  matter  of  plants 

through  which  photosynthesis  takes  place. 
CHLOROPLASTID.      The    special    protoplasmic    bodies    in    which 

chlorophyll,  or  functionally  similar  pigments,  resides. 
CHORD  ATE.    An  animal  whose  primary  axial  skeleton  consists  tem- 
porarily or  permanently  of  a  notochord.    All  Vertebrates  are 
Chordates. 
CHROMATIN.    A  deeply  staining  substance  characteristic  of  the 

nucleus,  forming  chromosomes,  etc.     See  Germ  Plasm. 
CHROMOMERE.    A  chromatin  granule  of  the  linear  series  which  con- 
stitute a  chromosome. 
CHROMOSOME.     One  of  the  deeply  staining  bodies  into  which  the 

chromatic  network  of  the  nucleus  becomes  visibly  resolved 

during  mitosis.    See  Germ  Plasm. 
CILIA.     Delicate  protoplasmic  projections  from  a  cell,  which  lash 

in  unison  and  propel  the  cell  in  the  water  (e.g.,  Paramecium),  or 

move  particles  over  the  cell  surface  (e.g.,  cells  lining  various  tubes 

in  multicellular  forms) . 
CLASS.    In  classification,  a  main  subdivision  of  a  phylum.     See 

Order. 
CLEAVAGE.    The  divisions  which  transform  the  egg  into  the  blastula 

stage  during  development. 
CLOACA.    A  cavity  at  the  posterior  end  of  the  Vertebrate  body,  into 

which  the  intestine,  urinary,  and  reproductive  ducts  open.     Not 

present  in  most  Mammals. 
COCHLEA.    The  portion  of  the  ear,  in  communication  with  the  sac- 

culus,  which  is  the  essential  organ  of  hearing  ill  the  higher 

Vertebrates. 
COELOM.     The  body  cavity,  lying  between  the  digestive  tract  and 

the  body  wall.    Lined  with  mesodermal  tissue. 
COELOMATE.     Possessing  a  coelom,  or  body  cavity;  as  in  all  the 

chief  groups  of  animals  above  the  Coelenterates.    The  latter  are 

acoelomate. 

COELOMIC  EPITHELIUM.    See  Peritoneum. 
COLLOID.    A  state  of  matter  in  which  a  substance  is  finelv  divided 


434  APPENDIX 

into  particles  larger  than  one  molecule  and  suspended  in  another 

substance. 
COLONY.      An  aggregation,  or  intimate  association  of  several  or 

many  individuals  to  form  a  superior  unit. 
COMBINATION.     Heritable  variation  due  to  recombinations  of  genes 

at  maturation  or  fertilization. 
CONJUGATION.     The  temporary  union  of  two  cells  during  which 

sexual  phenomena  occur;  e.g.,  in  Paramecium.     See  Endomixis. 
CONSERVATION  OF  ENERGY.     The  '  law '  that  the  total  energy  of  the 

universe  is  constant,  none  being  created  or  destroyed  but  merely 

transformed  from  one  form  to  another. 
CONTRACTILE  VACUOLE.    A  reservoir  in  unicellular  organisms  (e.g. 

Paramecium)  in  which  water  and  waste  products  of  metabolism 

collect  and  are  periodically  expelled  to  the  exterior. 
CORM.     A  solid  bulb-like  expansion  of  a  plant  stem  below  the  surface 

of  the  ground.     A  bulb  is  an  underground  storage  leaf  bud. 
COROLLA.     The  whorl  of  modified  leaves  immediately  within  the 

calyx  of  a  flower.     The  petals  collectively. 
CORTEX.     The  cylinder  between  the  outer  and  central  cylinder  in 

root  and  stem  of  the  higher  plants. 
COTYLEDON.    A  seed  leaf.     The  first  leaf  (in  monocotyledons)  or 

pair  of  leaves  (in  dicotyledons)  of  the  young  sporophyte  within 

the  seed. 

CRANIAL  NERVES.     Nerves  which  arise  from  the  brain. 
CRANIUM.     The  protective  case  enclosing  the  brain. 
CROSSING-OVER.     The  rearranging  of  linked  characters  as  a  result 

of  the  exchange  of  genes  during  synapsis  of  chromosomes. 
CRURA  CEREBRI.     Thickenings  of  ventral  surface  of  mid-brain. 
CRUSTACEA.  A  group  of  Arthropoda,  including  Crayfish,  Crabs,  etc. 
CUTICLE.    The  outermost  lifeless  layer  of  organisms. 
CYST.    A  resistant  envelope  formed  about  an  organism  (e.g.,  many 

Protozoa)  during  unfavorable  conditions  or  reproduction. 
CYTOLOGY.     The  science  of  cell  structure  and  function. 
CYTOPLASM.     The  protoplasm  of  a  cell  exclusive  of  the  nucleus. 
DECAY.    Chemical  decomposition  involving  putrefaction  or  other 

types  of  fermentation. 


GLOSSARY  435 

DENITRIFYING  BACTERIA.  Types  of  Bacteria  which  break  down 
compounds  of  nitrogen  and  set  free  the  nitrogen. 

DERMAL.  Pertaining  to  the  skin.  The  dermis  is  the  inner  layer  of 
the  Vertebrate  skin.  See  Epidermis. 

DIFFERENTIATION.  A  transformation  from  relative  homogeneity  to 
~  heterogeneity  r  involving;  the  production  of  specific  substances  or 
parts  from  a  general  substance  or  part.  Specialization. 

DIHYBRID.  The  progeny  of  parents  differing  in  regard  to  two  given 
characters. 

DIPLOID.  The  maximum  or  full  (duplex)  number  of  chromosomes 
which  occurs  during  the  life-history  of  a  given  species.  See 
Haploid. 

DIVISION  OF  LABOR.  Allocation  of  special  functions  to  special 
parts  which  cooperate  toward  the  unity  of  the  whole. 

DOMINANT  CHARACTER.  One  of  a  pair  of  alternative  characters 
which  appears  to  the  exclusion  of  the  other  (recessive)  character. 

DORSAL  AORTA.  Chief  artery  distributing  pure  blood  to  the  body. 
Ventral  aorta  carries  blood  from  heart  to  gill-arteries  in  Fishes. 

DUCTLESS  GLAND.  An  organ  whose  function  is  to  elaborate  and  se- 
crete a  hormone  directly  into  the  blood.  An  endocrine  gland. 

ECOLOGY.  The  study  of  the  relations  of  the  organism  to  environing 
conditions,  organic  and  inorganic. 

ECTODERM.  The  primary  tissue  comprising  the  surface  layer  of  cells 
in  the  gastrula;  its  derivatives  in  subsequent  stages  forming  the 
outer  part  of  the  skin,  nervous  system,  etc.  See  Germ  Layer. 

ECTOPLASM.  Modified  surface  layer  of  cytoplasm  of  a  cell.  See 
Endoplasm. 

EFFERENT  ROOT.  Ventral,  or  anterior,  root  of  certain  cranial 
and -all  spinal  nerves  through  which  motor  nerve  impulses  leave 
the  brain  and  spinal  cord.  See  Afferent  Root. 

EGG.     The  female  gamete.     Ovum. 

EMBRYOLOGY.  The  study  of  the  early  development  of  individual 
organisms. 

EMBRYO  SAC.     Megaspore  of  the  Flowering  Plants. 

EMULSOID.  A  state  in  which  one  liquid  is  divided  into  very  fine  drop- 
lets and  suspended  in  another  liquid  with  which  it  is  immiscible. 


436  APPENDIX 

ENCYSTMENT.    The  formation  of  a  resistant  covering,  or  cyst  wall, 

about  an  organism. 

ENDOCRINE  GLAND.    See  Ductless  Gland. 
ENDODERM.    The  primary  tissue  comprising  the  inner  layer  of  cells 

in  the  gastrula,  and  in  subsequent  stages  forming  the  lining  of 

the  essential  parts  of  the  digestive  tract  and  its  derivatives.    See 

Germ  Layer. 
ENDOMIXIS.     A  nuclear  reorganization  process  in  Protozoa,  e.g., 

Paramecium,  which  does  not  involve  the  cooperation  of  two 

cells  (as  in  conjugation)   and  therefore  is  without  synkaryon 

formation. 
ENDOPLASM.     The  inner  cytoplasm  surrounding  the  nucleus;  e.g., 

in  Paramecium.     See  Ectoplasm. 
ENDOPODITE.    The  inner  of  the  two  distal  parts  of  the  typical  bira- 

mous  Crustacean  appendage.     See  Protopodite  and  Exopodite. 
ENDOSKELETON.     An   internal   living   skeleton   affording   support 

and  protection,  as  well  as  levers  for  the  attachment  of  muscles. 

Characteristic  of  Vertebrates. 
ENDOSPERM.    A  tissue,  containing  reserve  food  materials,  formed 

within  the  embryo  sac. 
ENTERIC  CAVITY.    The  digestive  cavity  of  the  gastrula  stage,  and  of 

simple  Metazoa,  e.g.,  Hydra. 
ENZYMES.     Complex  chemical  substances  of  organisms  which  bring 

about  by  catalytic  action  many  of  the  chemical  processes  of  the 

body;  e.g.,  digestion. 

EPIDERMIS.    The  outer  cellular  layer  of  the  skin. 
EPIGENESIS.     Development  from  absolute  or  relative  simplicity  to 

complexity.     See  Preformation. 

EPITHELIUM.     A  layer  of  cells  covering  an  external  or  internal  sur- 
face, including  the  essential  secreting  cells  of  glands. 
EQUATION  DIVISION.    A  typical  division  of  the  nucleus  involving 

division  of  the  chromosomes.     See  Reduction  Division. 
EQUATORIAL  PLATE.     The  equator  of  the  spindle  with  its  group  of 

chromosomes  during  the  metaphase  of  mitosis. 
EUGENICS.    The  system  of  improving  the  human  race  by  breeding 

the  best.     "The  science  of  being  well  bom."    See  Euthenics. 


GLOSSARY  437 

EUSTACHIAN  TUBE.  Passage  connecting  the  Vertebrate  middle  ear 
with  the  pharynx.  Remnant  of  the  most  anterior  gill  slit,  rep- 
resented in  present-day  Sharks  by  the  'blow-hole,'  or  spiracle. 

EUTHENICS.  The  system  of  improving  the  human  race  by  good 
environment.  Sep.  Eugenics. 

EUTHERIA.  The  highest  of  the  three  subclasses  of  Mammals, 
including  all  the  familiar  forms-.  See  Appendix  I,  Classification. 

EVOLUTION,  ORGANIC.  The  accepted  theory  that  present-day 
organisms  are  the  result  of  descent  with  modification,  or  change, 
from  those  of  the  past.  The  word  'modification'  is  not  used  in 
the  technical  sense  employed  in  genetics.  See  Modifications. 

EXCRETION.  The  elimination  of  waste  products  of  metabolism. 
The  waste  products  themselves.  See  Secretion. 

EXOPODITE.  The  outer  of  the  two  distal  parts  of  the  typical,  bira- 
mous,  Crustacean  appendage.  See  Protopodite  and  Endopodite. 

EXOSKELETON.  A  non-living  external  skeleton  chiefly  for  protec- 
tion. The  characteristic  skeleton  of  Invertebrates,  e.g.,  Cray- 
fish. 

EXTERNAL  RECEPTORS.  Sense  organs  upon  the  surface  of  the  body. 
See  Internal  Receptors. 

EXTRACTED  DOMINANT.  A  homozygous  individual,  exhibiting 
the  dominant  character,  derived  from  heterozygous  (hybrid) 
parents. 

EXTRACTED  RECESSIVE.  An  individual  exhibiting  the  recessive 
character,  necessarily  homozygous,  derived  from  heterozygous 
(hybrid)  parents. 

FAMILY.  In  classification,  a  main  subdivision  of  an  order.  See 
Genus. 

FATS.  One  of  the  chief  groups  of  foodstuffs.  Organic  salts  con- 
sisting of  the  glycerol  radical  (C3H5),  the  basic  part,  combined 
with  a  fatty  acid.  E.g.,  mutton  tallow  is  chiefly  the  fat  Stearin 
(CsyHnuOe)  =  Glycerin  plus  Stearic  acid. 

FERMENTATION.  The  transformation  of  organic  substances  chiefly 
through  the  activity  of  ferments,  or  enzymes,  derived  from 
living  organisms.  See  Putrefaction. 

FERTILIZATION.    The  union  of  male  and  female  gametes,  especially 


438  APPENDIX 

their  nuclei  (pronuclei),  by  which  the  chromatin  complex  of  each 
is  arranged  to  form  the  composite  nucleus  of  the  zygote. 

FLAGELLUM.  A  whip-like  prolongation  of  the  cytoplasm,  the  move- 
ments of  which  usually  effect  the  locomotion  of  the  cell;  e.g., 
Sphaerella. 

FLOWER.  A  group  of  sporophylls  and  accessory  structures,  as  in  the 
Flowering  Plants. 

FLUCTUATIONS.  Relatively  slight  variations  always  found  in  organ- 
isms; may  be  either 'modifications  or  combinations. 

FOETAL  MEMBRANE.  The  embryo  of  the  higher  Mammals  before 
birth  lies  in  "the  uterus  of  the  mother  enclosed  in  a  series  of 
membranes  the  outer  one  of  which  is  in  intimate  contact  with 
the  uterine  wall  at  one  or  more  points  to  form  the  placenta. 

FOVEA  CENTRALIS.  A  slight  depression  at  the  posterior  end  of 
the  optical  axis  of  the  eyeball.  The  center  of  distinct  vision. 

FROND.     Fern  leaf,  usually  both  vegetative  and  spore-producing. 

FRUIT.  The  ripened  ovule  case  and  contents,  together  with  any 
structures  which  by  adhesion  become  an  integral  part  of  it. 

GALL  BLADDER.  Receptacle  near  the  liver  for  the  temporary 
storage  of  "bile. 

GAMETANGIUM.  A  gamete-producing  organ,  especially  in  the  lower 
plants. 

GAMETE.  A  cell  which  unites  with  another  at  fertilization  to  form 
a  zygote.  Egg  or  sperm. 

GAMETOPHYTE.     The  sexual,  gamete-bearing  generation  in  plants. 

GANGLION.  A  .group  of  nerve  cells,  chiefly  the  cell  bodies,  with 
supporting  cells. 

GASTRIC  VACUOLE.  A  droplet  of  fluid  enclosing  ingested  food,  in 
which  digestion  occurs;  e.g.,  in  Paramecium. 

GASTROLITHS.  Calcareous  bodies  found  at  certain  times  in  the 
lateral  walls  of  the  stomach  of  the  Crayfish.  Probably  represent 
the  storage  of  material  for  the  exoskeleton. 

GASTRULA.  A  stage  in  animal  development  in  which  the  embryo 
consists  of  a  two-layered  sac,  ectoderm  and  endoderm,  enclosing 
the  enteric  cavity  which  opens  to  the  exterior  by  the  blastopore. 

GEL.    A  colloid  which  is  more  or  less  rigid. 


GLOSSARY  439 

GENE.  A  factor  or  element  in  the  chromosomes  of  the  germ  cells 
which  conditions  a  character  of  an  organism. 

GENETICS.    The  science  of  heredity. 

GENOTYPE.  The  fundamental  hereditary  constitution  of  an  organ- 
ism or  group  of  organisms.  The  gene  complex  of  an  organism. 
See  Phenotype. 

GENUS.  In  classification,  a  main  subdivision  of  a  family.  See 
Species. 

GERMINAL  CONTINUITY.  The  concept  of  an  unbroken  stream  of 
germ  plasm  from  the  beginning  of  life,  from  which  each  genera- 
tion is  derived. 

GERM  LAYER.  A  primary  tissue  (ectoderm,  endoderm,  or  meso- 
derm)  in  the  embryo  from  which  the  tissues  and  organs  of  the 
adult  animal  develop. 

GERM  LAYER  THEORY.  The  doctrine  that  the  germ  layers  are 
fundamentally  similar  throughout  the  Metazoa  and  that  homolo- 
gous structures  in  various  animals  are  derived  during  ontogeny 
from  the  same  germ  layer. 

GERM  PLASM.  The  physical  basis  of  inheritance.  The  chromatin 
which  forms  the  specific  bond  of  continuity  between  parent  and 
offspring.  Contrasted  with  soma  or  somatoplasm. 

GILL  SLITS.  Paired  lateral  openings  leading  from  the  anterior  end 
of  the  alimentary  canal  to  the  exterior  for  the  exit  of  the  respira- 
tory current  of  water.  Permanent  or  embryonic  characters 
of  Vertebrates.  Branchial  clefts. 

GLAND.  One  cell  or  a  group  of  many  epithelial  cells  which  elaborate 
certain  materials  and  then  secrete  the  product  for  the  use  of  the 
organism. 

GLOTTIS.  The  opening  from  the  pharynx  into  the  tube  (trachea) 
leading  to  the  lungs. 

GONAD.  An  organ  in  which  the  germ  cells  develop.  Ovary  or 
test  is. 

GREEN  GLANDS.  Excretory  organs  (nephridia)  of  the  Crayfish 
and  its  allies. 

HAPLOID.  The  reduced  (one-half)  number  (simplex  group)  of 
chromosomes.  See  Diploid. 


440  APPENDIX 

HAUSTORIA.  Sucker-like  absorbing  organs  of  parasitic  plants;  e.g., 
Dodder. 

HEPATIC  PORTAL  SYSTEM.  Non-oxygenated  but  food-laden  blood 
from  digestive  tract  to  the  liver  via  hepatic  portal  vein.  Oxygen- 
ated blood  reaches  liver  via  hepatic  artery.  All  leaves  via 
hepatic  vein.  Thus  there  is  a  double  blood  supply  to  liver  in 
all  Vertebrates. 

HEREDITY.  The  transmission  of  characters  from  parent  to  off- 
spring through  the  germ  cells. 

HERMAPHRODITE.  An  organism  bearing  both  male  and  female 
reproductive  organs;  e.g.,  Hydra  and  Earthworm. 

HETEROSPORY.  The  condition  of  producing  two  kinds  of  spores, 
megaspores  and  microspores,  as  in  the  higher  plants. 

HETEROZYGOUS.  Producing  gametes  which  fall  into  two  numeri- 
cally equal  classes  with  respect  to  the  genes  (allelomorphs)  for  a 
pair  of  alternative  characters.  See  Homozygous. 

HISTOLOGY.  The  science  which  treats  of  animal  and  plant  tissues. 
Microscopic  anatomy. 

HOLOPHYTIC.  Type  of  nutrition  involving  photosynthesis.  Char- 
acteristic of  green  plants.  See  Holozoic  and  Saprophytic. 

HOLOZOIC.  Type  of  nutrition  involving  the  ingestion  of  solid  food. 
Characteristic  of  animals.  See  Holophytic  and  Saprophytic. 

HOMOLOGOUS  CHROMOSOMES.  The  members  of  a  pair  of  chromo- 
somes, of  a  duplex  group,  one  paternal  and  the  other  maternal  in 
origin,  which  bear  the  same  or  allelomorphic  genes.  See  Synap- 
tic  Mates. 

HOMOLOGOUS  GENES.  Genes  similarly  situated  on  homologous 
chromosomes.  See  Allelomorph. 

HOMOLOGY.  Fundamental  structural  similarity,  regardless  of  func- 
tion, due  to  descent  from  a  common  form. 

HOMOTHERMAL.  Animals  provided  with  a  mechanism  which  main- 
tains the  body  at  a  practically  constant  temperature,  usually 
higher  than  that  of  the  environment.  E.g.,  the  'warm-blooded' 
animals,  or  Birds  and  Mammals. 

HOMOZYGOUS.  Producing  gametes  all  of  which  bear  the  gene  for 
one  of  a  pair  of  alternative  characters.  See  Heterozygous. 


GLOSSARY  441 

HORMONE.  An  internal  secretion,  usually  from  a  ductless  gland, 
which  is  distributed  by  the  blood  and  influences  the  activities 
of  one  or  more  parts  of  the  body. 

HYALINE.    Pellucid  or  glassy. 

HYBRID.  The  progeny  of  parents  which  differ  in  regard  to  one  or 
more  characters. 

HYDROIDS.  A  group  of  animals  allied  to  Hydra,  exhibiting  alterna- 
tion of  generations. 

IMMUNITY.  Resistance  of  the  body  to  infection  by  disease-produc- 
ing organisms.  Exemption  from  disease. 

INFUNDIBULUM.  A  funnel-like  outgrowth  from  the  ventral  wall  of 
the  diencephalon.  See  Pituitary  Body. 

INTERCELLULAR  DIGESTION.  Digestion  by  the  secretion  of  enzymes 
into  a  digestive  cavity;  e.g.,  in  Earthworm  and  Man.  See  Intra- 
cellular  digestion. 

INTERNAL  RECEPTORS.  Sense  organs  within  the  body.  See  Exter- 
nal receptors. 

INTERNAL  SECRETION.    See  Hormone  and  Ductless  Gland. 

INTESTINE.  Portion  of  the  alimentary  canal  from  pyloric  end  of 
stomach  to  anus.  Divided  into  small  and  large  intestine. 

INTRACELLULAR  DIGESTION.  Digestion  of  food  within  the  cell  it- 
self; e.g.,  in  Paramecium  and  to  some  extent  in  the  endoderm 
cells  of  Hydra.  See  Intercellular  digestion. 

INTUSSUSCEPTION.  Interstitial  growth  by  the  addition  of  new 
particles  throughout  the  whole  mass  of  protoplasm.  Contrasted 
with  growth  by  accretion,  or  the  deposition  of  particles  on  the 
surface  as  in  crystals. 

INVAGINATION.  Sinking  or  growing  in  of  a  portion  of  the  surface  of 
a  hollow  body;  e.g.,  during  transformation  of  blastula  to  gastrula. 

INVERTEBRATE  .  An  animal  without  a  notochord  or  a  vertebral  column. 

IRRITABILITY.  The  power  of  responding  to  stimuli,  exhibited  by  all 
protoplasm. 

KARYOLYMPH.  The  more  fluid  material  of  the  nucleus  in  contrast 
with  the  linin  and  chromatin. 

KARYOSOME.  An  aggregation  of  part  of  the  chromatin  material 
within  the  nucleus.  A  'net-knot7.  See  Nucleolus. 


442  APPENDIX 

KATABOLISM.    The  destructive  phase  of  metabolism.    See  Anabo- 

lism. 

KINETIC  ENERGY.  Energy  possessed  by  virtue  of  motion.  E.g., 
union  of  C  with  02  transforms  chemical  potential  energy  into 
kinetic  energy,  i.e.,  heat,  etc.  See  Potential  Energy. 

LAMINA.    The  blade  of  a  leaf. 

LARVA.  An  immature  stage  in  the  life  history  of  certain  animals, 
usually  active  arid  differing  widely  in  appearance  from  the  adult. 
E.g.,  caterpillar  of  a  Butterfly,  tadpole  of  Frog. 

LENTICELS.  Openings  on  the  outer  surface  of  the  bark  which  per- 
mit a  slight  amount  of  gaseous  interchange.  Arise  as  stomata  in 
the  young  shoot. 

LININ.  The  material  of  the  reticulum  of  the  nucleus,  upon  and 
through  which  the  chromatin  appears  to  be  distributed  in  the 
resting  cell.  The  representative  within  the  nucleus  of  the  gen- 
eral cytoplasmic  reticulum. 

LINKAGE.  Tendency  for  certain  characters  to  be  inherited  in 
groups,  probably  because  the  genes  for  the  characters  are  closely 
associated  on  the  same  chromosome. 

LYMPH.  Essentially  plasma  and  white  blood  corpuscles  which  have 
passed  through  the  capillary  walls  to  supply  the  milieu  of  the 
tissue  cells. 

MACRONUCLEUS.  The  large  'vegetative'  nucleus  in  Infusoria  with 
dimorphic  nuclei;  e.g.,  in  Paramecium.  See  Micronucleus. 

MANDIBLES.  Jaws.  The  third  pair  of  appendages  of  the  head  of 
the  Crayfish. 

MATURATION.  Final  stages  in  the  formation  of  the  germ  cells,  in- 
volving chromosome  reduction. 

MAXILLIPEDS.  The  three  posterior  pairs  of  appendages  of  the 
head  of  the  Crayfish. 

MECHANISM.  The  doctrine  that  the  phenomena  of  life  are  inter- 
pretable  in  terms  of  the  laws  of  matter  and  energy  which  hold 
in  the  realm  of  the  non-living.  See  Vitalism. 

MEDUSA.     Sexual,  gonad-bearing  generation  of  hydra-like  animals, 
the  Hydroids. 

MEGASPORANGIUM.    A  sporangium  which  bears  megaspores. 


GLOSSARY  443 

MEGASPORE.  The  large  spore  which  in  heterosporous  plants  forms 
a  female  gametophyte. 

MEGASPOROPHYLL.  A  modified  leaf  of  a  heterosporous  sporophyte 
which  produces  megaspores.  A  carpel. 

MERISTEM.  Formative  tissue  with  rapidly  dividing  cells,  as  in  cam- 
bium and  growing  points  of  plants. 

MESODERM.  A  primary  tissue,  or  germ  layer,  of  animals  which 
develops  between  ectoderm  and  endoderm.  See  Germ  Layer. 

MESOGLOEA.  The  non-cellular  layer  between  ectoderm  and  endo- 
derm in  Hydra  and  other  Coelenterates. 

MESOPHYLL.  Tissue  of  the  leaf,  between  upper  and  lower  epider- 
mis, exclusive  of  the  vascular  bundles  (veins). 

METABOLISM.  The  sum  of  the  chemical  processes  in  organisms, 
involving  the  building  up  and  breaking  down  of  the  living  matter. 
See  Anabolism  and  Katabolism. 

METAGENESIS.     Alternation  of  generations,  as  in  Obelia. 

METAMERE.  One  of  the  series  of  similar  parts,  or  segments,  of  the 
body;  e.g.,  in  the  Earthworm  and  Crayfish  and,  in  highly  modi- 
fied form,  throughout  the  Vertebrates. 

METAMORPHOSIS.  A  more  or  less  abrupt  transition  from  one  devel- 
opmental stage  to  another.  E.g.,  in  Insects. 

METAPHASE.  Climax  of  mitosis  involving  the  separation  of  the 
halves  of  the  longitudinally  split  chromosomes  arranged  in  the 
equatorial  plate.  See  Anaphase. 

METAPH YTA .     Multicdli.il ar  plants . 

METAPLASM.  Lifeless  inclusions  in  cytoplasm;  e.g.,  yolk  granules, 
etc. 

METAZOA.     Multicellular  animals. 

MICRONUCLEUS.  The  small  'germinal'  nucleus  in  Infusoria  with 
dimorphic  nuclei;  e.g.,  Paramecium  caudatum  has  one,  and  P. 
aurelia  and  P.  calkinsi  have  two  micronuclei.  See  Macro- 
nucleus. 

MICROSPORANGIUM.  A  sporangium  which  bears  microspores;  e.g., 
pollen  sacs  in  anther  of  stamen. 

MICROSPORE.  The  small  spore,  of  heterosporous  plants,  which 
forms  a  male  gametophyte.  A  pollen  grain. 


444  APPENDIX 

MICROSPOROPHYLL.  A  modified  leaf,  of  a  heterosporous  sporo- 
phyte,  which  produces  microspores.  A  stamen. 

MITOSIS.    The  typical  process  of  cell  division. 

MODIFICATIONS.  In  genetics:  changes  in  the  soma  due  to  environ- 
mental influences;  so-called  acquired  characters  are  modifica- 
tions. In  evolution :  signifies  '  change ' ;  no  technical  connotation. 

MONOHYBRID.  The  progeny  of  parents  differing  in  regard  to  one 
given  character. 

MORPHOGENESIS.  The  origin  of  the  form  and  structure  of  an  organ- 
ism during  ontogeny. 

MORPHOLOGY.     The  science  of  the  form  of  animals  and  planter 

MOSAIC  INHERITANCE.  Inheritance  of  a  character  in  part  from  each 
parent  but  without  blending. 

MUTATION.  A  heritable  variation  due  to  a  fundamental  change  in 
the  constitution  of  the  germ  plasm,  independent  of  the  normal 
processes  of  segregation  and  crossing-over. 

MYOTOMES.  Muscle  segments  in  body  wall  of  lower  Vertebrates 
and  embryos  of  higher  forms. 

NATURAL  SELECTION.  The  processes  occurring  in  nature  which 
result  in  the  " survival  of  the  fittest"  individuals  and  the  elimi- 
nation of  those  less  adapted  to  the  conditions  imposed  by  their 
environment  and  mode  of  life. 

NEPHRIDIUM.    An  excretory  organ;  e.g.,  in  Earthworm. 

NEPHROSTOME.    Coelomic  opening  or  funnel  of  a  nephridium. 

NERVE.  Essentially  a  group  or  cable  of  parallel  nerve  fibers  bound 
together.  See  Axon. 

NEURAL  CANAL.  The  tube  in  which  the  brain  and  spinal  cord  lie. 
Formed  by  the  neural  arches  and  centra  of  the  vertebrae. 

NEURAL  TUBE.  A  tube  derived  from  the  ectoderm  and  forming 
the  brain  and  spinal  cord  in  Vertebrates. 

NEURON.  A  nerve  cell,  comprising  cell  body  and  cytoplasmic  pro- 
cesses. See  Axon. 

NITRIFYING  BACTERIA.  Types  of  Bacteria  which,  in  the  process  of 
their  nutrition,  change  ammonia  (NH3)  into  compounds  with  the 
N(>2  radical  (nitrites),  and  change  nitrites  into  compounds  with 
the  N03  radical  (nitrates.) 


GLOSSARY  445 

NITROGEN-FIXING  BACTERIA.  Types  of  Bacteria  which  take  free 
atmospheric  nitrogen  and  combine  it  with  oxygen  so  that  nitrates 
available  for  green  plants  are  formed.  Found  in  the  soil  and 
in  tubercles  on  rootlets  of  various  leguminous  plants. 

NOTOCHORD.  An  axial  cord  of  cells  about  which  the  backbone  is 
formed.  Gradually  replaced  by  the  centra  of  the  vertebrae  in 
the  ascending  series  of  Vertebrates. 

NUCELLUS.  The  megasporangium  of  Flowering  Plants.  See  Ovule 
and  Embryo  sac. 

NUCLEOLUS.  A  spherical  body  of  achromatic  material  within  the 
nucleus.  Plasmosome.  See  Karyosome. 

NUCLEUS.  A  specialized  protoplasmic  body  in  all  typical  cells. 
Most  characteristic  element  is  chromatin. 

OESOPHAGUS.     Narrow  tube  leading  from  pharynx  to  stomach. 

OLFACTORY.     Relating  to  the  sense  of  smell. 

ONTOGENY.  The  developmental  history  of  the  individual.  See 
Phylogeny. 

OOCYTE.     The  ovarian  egg  before  maturation. 

OOGENESIS.  The  development  of  the  mature  egg  from  a  primordial 
germ  cell. 

OPTIC  LOBES.    Thickenings  of  the  dorsal  surface  of  the  mid-brain. 

ORDER.   In  classification,  a  main  subdivision  of  a  class.   See  Family. 

ORGAN.  A  complex  of  tissues  for  the  performance  of  a  certain  func- 
tion; e.g.,  the  heart. 

OSMOSIS.  Diffusion  of  dissolved  substances  through  a  semi-perme- 
able membrane.  Osmotic  pressure  may  be  considered  as  a  result 
of  the  inhibited  power  of  diffusion  of  a  dissolved  substance  —  in- 
hibited because  the  membrane  is  semi-permeable.  The  physical 
phenomena  of  diffusion  and  osmosis  are  complicated  in  living 
cells  by  the  fact  that  their  limiting  surfaces  may  function  now  as 
permeable  and  again  as  semi-permeable  membranes,  i.e.,  per- 
mitting water  but  not  the  substance  in  solution  to  pass 
through. 

OSTEOLOGY.    The  study  of  the  Vertebrate  skeleton. 

OVARY.  The  definitive  female  reproductive  organ  in  which  "fcfoe 
gametes  (eggs)  develop. 


446  APPENDIX 

OVULE.  The  body  which  after  fertilization  of  the  egg  becomes  a 
seed.  The  ovule  consists  of  protective  envelopes  (integument) 
enclosing  the  nucellus  (megasporangium)  with  the  embryo  sac 
(megaspore) . 

OVULE  CASE.  The  base  of  the  pistil  in  which  ovules  arise. 
"Ovary." 

OVUM.    Egg.    Female  gamete. 

OXIDATION.  The  combination  of  any  substance  or  its  constituent 
parts  with  oxygen. 

PALEONTOLOGY.  The  science  of  extinct  animals  and  plants  repre- 
sented by  fossil  remains. 

PARASITE.  An  organism  which  secures  its  livelihood  directly  at  the 
expense  of  another  living  organism,  on  or  in  whose  .body  it  lives. 

PARTHENOGENESIS.    Development  of  an  egg  without  fertilization. 

PATHOGENIC.  Disease-producing,  especially  in  regard  to  the  rela- 
tion of  a  parasite  to  its  host. 

PEDUNCLE.    Stalk  of  a  flower;  represents  the  floral  branch. 

PENTADACTYL.  Having  five  fingers  or  toes;  typical  Vertebrate 
limb. 

PERIANTH.     Collective  term  for  calyx  and  corolla. 

PERICARDIUM.  Peritoneum  lining  the  pericardial  cavity  containing 
the  heart. 

PERISTALSIS.  Rhythmical  contractions  of  the  wall  of  the  alimen- 
tary canal  which  forces  the  food  along. 

PERITONEUM.  Membrane  lining  coelom  of  Vertebrates.  Consists 
of  an  outer  layer  of  connective  tissue  next  to  the  muscles  of  body 
wall  and  an  inner  layer  of  coelomic  epithelium  which  forms  the 
innermost  layer  of  body  wall. 

PETAL.    One  of  the  leaves  of  the  corolla  of  a  flower. 

PETIOLE.    A  leaf  stalk. 

PHARYNX.  Region  of  alimentary  canal  between  buccal  cavity,  or 
mouth,  and  oesophagus.  Throat. 

PHENOTYPE.  The  somatic,  or  expressed,  characters  of  an  organism 
or  group  of  organisms  irrespective  of  those  potential  in  their  germ 
cells.  See  Genotype. 

PHLOEM.    The  outer  part  of  a  vascular  bundle.    'Inner  bark.' 


GLOSSARY  447 

PHOTOSYNTHESIS.  Process  by  which  complex  compounds  are  built 
up  from  simple  elements  through  the  energy  of  sunlight  absorbed 
by  chlorophyll,  or  a  functionally  similar  pigment. 

PHYLOGENY.    The  ancestral  history  of  the  race.    See  Ontogeny. 

PHYLUM.  In  classification,  a  main  subdivision  of  the  animal  or 
plant  kingdom.  See  Class. 

PHYSIOLOGY.  The  study  of  the  functions  of  animals  and  plants. 
The  mechanical  and  chemical  engineering  of  organisms. 

PINEAL  BODY.  An  outgrowth  from  the  upper  wall  of  the  diencepha- 
lon.  The  vestige  of  an  additional  pair  of  eyes  possessed  by  the 
ancestors  of  existing  Vertebrates.  Fcssibly  functions  as  an  en- 
docrine gland  in  Mammals.  Brow-spot  of  Frog. 

PISTIL.  Organ  of  the  flower,  composed  of  ovule  case,  style,  and 
stigma.  See  Carpel. 

PITH.  Middle  part  of  the  central  cylinder  of  a  plant  shoot.  Func- 
tions largely  for  the  storage  of  water  and  food. 

PITH  RAYS.  Extensions  of  the  pith  which  radiate  between  the 
vascular  bundles  to  the  bark.  Medullary  rays. 

PITUITARY  BODY.  An  ingrowth  of  the  ectodermal  tissue  above  the 
mouth  and  the  tip  of  the  infundibulum  from  the  ventral  wall 
of  the  diencephalon  unite  to  form  a  gland-like  structure  (pitui- 
tary body  or  hypophysis) . 

PLACENTA.  A  Mammalian  organ  adapted  for  the  interchange  of  all 
nutritive,  respiratory,  and  excretory  materials  between  the 
embryo  (foetus)  and  mother.  It  also  serves  as  an  organ  of 
attachment.  In  the  higher  Mammals  it  is  composed  of  both 
foetal  and  maternal  tissues.  See  Umbilical  Cord. 

PLASMA.     Liquid  portion  of  the  blood. 

PLEXUS.  Intercommunication  of  the  fibers  from  one  nerve  with 
those  of  another  to  form  a  network  of  nerves;  e.g.,  branchial  and 
sciatic  plexus. 

POLAR  BODIES.  Tiny  abortive  cells  arising,  by  division,  from  the 
egg  during  maturation. 

POLE  CELLS.  Two  cells  which  give  rise  to  the  mesoderm  in  the 
development  of  the  Earthworm  and  its  allies. 

POLLEN.    The  microspores  of  Flowering  Plants. 


448  APPENDIX 

POLLINATION.  The  transference  of  pollen  to  the  stigma  of  the  pis- 
til in  higher  Flowering  Plants. 

POLYHYBRID.  The  progeny  of  parents  which  differ  in  regard  to 
more  than  three  given  characters. 

POLYMOEPHISM.  Occurrence  of  several  types  of  individuals  during 
the  life  history,  or  composing  a  colony;  e.g.,  in  some  Hydroids. 

POTENTIAL  ENERGY.  Energy  possessed  by  virtue  of  stresses,  i.e., 
two  forces  in  equilibrium.  Criterion  is  work  done  against  any 
restoring  force;  e.g.,  kinetic  energy  of  sunlight  through  agency 
of  chlorophyll  separates  C02  into  C  and  02  and  thereupon  is 
represented  by  an  equal  amount  of  chemical  potential  energy. 
Restoring  force  is  here  chemical  affinity.  Similarly  a  raised 
weight  possesses  gravitational  potential  energy  in  amount 
equal  to  kinetic  energy  expended  in  raising  it.  See  Kinetic 
Energy  and  Conservation  of  Energy. 

PREFORMATION.  The  abandoned  doctrine  that  development  is  es- 
sentially an  unfolding  of  an  individual  ready-formed  in  the  germ. 
See  Epigcnesis. 

PRONEPHROS.     Primitive  kidney  of  Vertebrates. 

PRONUCLEI.  The  nuclei  of  the  male  and  female  gametes  ready  to 
unite  at  fertilization. 

PROPHASE.  Preparatory  changes  during  mitosis  leading  to  the  dis- 
position of  the  chromosomes  in  the  center  of  the  cell  (equatorial 
plate)  ready  for  division.  See  Metaphase. 

PROSTOMIUM.  A  lobe  which  projects  from  the  first  segment  of  the 
body  of  the  Earthworm  and  forms  an  upper  lip. 

PROTEIN.  A  class  of  complex  chemical  molecules,  containing  nitro- 
gen, which  form  the  chief  characteristic  constituent  of  proto- 
plasm. 

PROTHALLUS.     The  gametophyte  of  Ferns. 

PROTISTA.     Protophyta  and  Protozoa;  all  unicellular  organisms. 

PROTONEMA.  A  filamentous  growth  from  a  Moss  spore  which  gives 
rise  to  the  leafy  Moss  plant. 

PROTOPHYTA.     Unicellular  plants.     See  Protista. 

PROTOPLASM.     The  physical  basis  of  life.     Living  matter. 

PROTOPLAST.    The  cell  exclusive  of  the  cell  wall,  especially  in  plants. 


GLOSSARY  449 

PROTOPODITE.  The  basal  portion  of  the  typical  Crustacean  ap- 
pendage from  which  arise  the  endopodite  and  exopodite. 

PROTOZOA.     Unicellular  animals. 

PURE  LINE.  A  group  of  individuals  bearing  identical  genes, 
derived  from  a  common  homozygous  ancestor. 

PUTREFACTION.  The  simplification  of  nitrogenous  compounds, 
such  as  proteins,  chiefly  through  the  action  of  enzymes  of  living 
organisms.  See  Fermentation. 

PYLORIC  VALVE.  Muscular  constriction  between  stomach  and 
small  intestine. 

RECAPITULATION  THEORY.  Doctrine  that  individual  development 
(ontogeny)  repeats  in  abbreviated  and  modified  form  the  develop- 
ment of  the  race  (phylogeny).  So-called  biogenetic  law. 

RECESSIVE  CHARACTER.    See  Dominant  character. 

REDUCTION.  The  halving  of  the  chromosome  number  during 
maturation.  Transformation  of  duplex  into  simplex  group. 

REDUCTION  DIVISION.  The  division  during  spermatogenesis  and 
oogenesis  which  separates  synaptic  mates  and  reduces  the 
chromosome  number  one  half.  The  mechanism  of  segregation. 

REFLEXES.  Relatively  simple  and  essentially  automatic  responses. 
Merge  into  instincts  which  are  the  most  complex  reactions 
made  without  learning. 

REGENERATION.  The  power  of  replacement  of  parts  which  have 
been  lost  through  mutilations  or  otherwise. 

RENAL  PORTAL  SYSTEM.  Blood  ('impure')  from  posterior  part  of 
the  body  to  kidneys  via  renal  portal  vein.  Oxygenated  blood  to 
kidneys  via  renal  artery.  Thus  in  animals  with  the  renal  portal 
system  there  is  a  double  blood  supply  to  the  kidneys.  Present 
in  Fishes,  Amphibians,  and  Reptiles;  vestigial  in  Birds;  absent 
in  Mammals. 

REPRODUCTION.  The  power  of  living  matter  to  reproduce  itself. 
Protoplasmic  growth  resulting  in  cell  division. 

RESPIRATION.  Essentially  the  securing  of  energy  from  fnodr  involv- 
_  ing  the  exchange  of  carbon  dioxide  for  oxygen  by  protoplasm. 

RESPONSE.  Any  change  in  the  activity  oi  protoplasm,  and  therefore 
of  an  organism  as  a  whole,  as  the  result  of  a  stimulus. 


450  APPENDIX 

RESTING  CELL.     One  which  is  not  undergoing  mitosis. 

RETINA.  Actual  percipient  part  of  the  eye  by  virtue  of  a  sensory 
layer  which  is  stimulated  by  light  rays. 

REVERSION.  The  appearance  of  a  distant  ancestral  character  in  an 
individual.  See  Atavism. 

RHIZOID.  A  root-like  filament  in  lower  plants;  e.g.,  in  Mosses  and 
prothallus  of  Ferns. 

RHIZOME.  Prostrate  underground  stem;  e.g.,  in  sporophyte  of  com- 
mon Ferns. 

ROOT  HAIRS.  Prolongations  of  epidermal  cells  just  above  the  grow- 
ing point  of  roots  which  afford  surface  for  intake  of  water  and 
solutes. 

ROSTRUM.  The  anterior  pointed  extension  of  the  exoskeleton  of  the 
Crayfish  and  its  allies. 

ROTIFERA.  Microscopic,  aquatic,  multicellular  animals.  Wheel 
animalcules. 

RUSTS.  Fungi  which  are  destructive  parasites  of  the  higher  plants; 
e.g.,  the  Wheat  Rust. 

SACCULUS.  The  anterior  sac  of  the  labyrinth  of  the  ear,  a  derivative 
of  which  becomes  the  cochlea  in  higher  Vertebrates. 

SAPROPHYTIC.  Type  of  nutrition  involving  the  absorption  of  com- 
plex products  of  organic  decomposition;  e.g.,  in  many  groups  of 
Bacteria  and  other  Fungi,  as  well  as  various  species  of  lower 
animals.  See  Holozoic  and  Holophytic. 

SEBACEOUS  GLANDS.  Glands  which  elaborate  a  fatty  substance 
(sebum)  and  secrete  it  in  the  hair  follicles. 

SECRETION.  A  substance  elaborated  by  glandular  epithelium;  or 
the  process  involved.  See  Gland  and  Excretion. 

SEED.  An  embryo  sporophyte  supplied  with  food  and  protective 
envelopes. 

SEGREGATION.  The  distribution  of  contrasting  genes  (allelomorphs) 
to  separate  cells  during  the  maturation  of  the  germ  cells  in  a 
heterozygous  individual  (hybrid). 

SEMICIRCULAR  CANALS.  Portion  of  the  Vertebrate  ear  devoted  to 
equilibrium. 

SEMINAL  RECEPTACLES.    Globular  sacs  within  the  body  cavity  of 


GLOSSARY  451 

the  Earthworm,  which  receive  the  sperm  from  another  worm 
and  retain  them  until  fertilization  is  to  occur. 

SEPAL.    A  leaf  of  the  calyx  of  a  flower. 

SEPTA.  The  partitions  which  divide  the  coelom  of  the  Earthworm 
into  a  series  of  chambers,  or  metameres. 

SERIAL  HOMOLOGY.  Homology  of  a  structure  of  an  organism  with 
another  of  the  same  organism;  e.g.,  appendages  of  the  Crayfish, 
fore-  and  hind-limbs  of  Vertebrates. 

SETAE.  Bristle-like  structures  which  protrude  from  the  body  wall 
of  the  Earthworm  and  aid  in  locomotion. 

SEX  CHROMOSOME.  The  odd,  X,  or  accessory  chromosome  which 
bears  the  differential  gene  for  sex. 

SEX-LINKED  CHARACTERS.  Characters  represented  by  genes  on 
the  sex  chromosomes. 

SHOOT.    Stem  and  leaves  as  contrasted  with  the  root. 

SIMPLEX  CHARACTER.  The  result  of  a  determiner,  or  gene,  from  one 
parent  only. 

SOL.     A  colloid  which  is  highly  fluid. 

SOMA.     Body  tissue  (somatoplasm)  in  contrast  with  germinal  tissue. 

SPECIAL  CREATION.  Abandoned  doctrine  that. each  species  was 
specially  created.  Implies  fixity  of  species.  See  Evolution. 

SPECIES.  In  classification,  the  main  subdivision  cf  a  genus.  A 
group  of  individuals  which  do  not  differ  from  one  another  in 
excess  of  the  limits  of  "individual  diversity,"  actual  or  as- 
sumed. 

SPERM.     Male  gamete.     Spermatozoon. 

SPERMATID.  Male  germ  cells  after  the  final  maturation  division 
but  before  assuming  the  typical  form. of  the  ripe  sperm. 

SPERMATOCYTES.  Cells  arising  from  the  spermatogonia.  Primary 
spermatocyte  arises  by  growth  from  the  last  generation  of 
spermatogonia.  Primary  divides  to  form  two  secondary 
spermatocytes. 

SPERMATOGENESIS.  The  development  of  the  sperm  from  a  primor- 
dial germ  cell. 

SPERMATOPHYTES.  Plants  bearing  true  seeds.  Seed  Plants.  Flow- 
ering Plants.  Phanerogams. 


452  APPENDIX 

SPINDLE.  The  'fiber-like'  apparatus  between  the  centrosomes 
during  mitosis. 

SPIREME.  The  linear  arrangement  of  the  chromosomes  frequently 
observed  during  mitosis. 

SPLEEN.  A  vascular  ductless  organ  of  most  Vertebrates,  usually 
situated  near  the  stomach,  which  produces  certain  changes  in 
the  blood. 

SPONTANEOUS  GENERATION.    See  Abiogenesis. 

SPORANGIUM.    A  spore-producing  structure  on  a  sporophyll. 

SPORE.  A  cell,  liberated  from  the  parent,  which  gives  rise  without 
fertilization  to  a  new  individual.  The  resistant  phase  assumed 
by  certain  unicellular  organisms;  e.g.,  Bacteria. 

SPOROPHYLL.    A  leaf  which  bears  sporangia. 

SPOROPHYTE.  Spore-bearing  (asexual)  generation  in  plants  exhibit- 
ing alternation  of  generations. 

SPORULATION.  Occurrence  of  several  simultaneous  divisions  by 
which  a  unicellular  organism  is  resolved  into  many  smaller  cells. 

STAMEN.  The  pollen-bearing  organ  in  Flowering  Plants.  A  micro- 
sporophyll.  See  Anther. 

STELE.  The  central  cylinder  of  root  and  stem,  formed  of  united  vas- 
cular bundles,  in  the  highest  Flowering  Plants. 

STIGMA.  The  tip  of  the  pistil  adapted  to  receive  the  pollen  and  pro- 
vide for  its  germination. 

STIMULUS.  Any  condition  which  calls  forth  a  response  from  living 
matter. 

STIPULES.  Pair  of  appendages  frequently  occurring  at  the  point 
(leaf  base)  where  the  petiole  joins  the  stem. 

STOMATA.  Openings  through  the  epidermis  of  a  leaf  for  the  inter- 
change of  gases  and  exit  of  water  vapor.  The  'stomatic  ap- 
paratus' comprises  the  stoma  and  its  guard  cells. 

STYLE.     An  elongation  of  a  pistil  which  bears  the  stigma. 

SYMBIOSIS.  The  association  of  two  species  in  a  practically  obliga- 
tory and  mutually  advantageous  partnership;  e.g.,  Lichens. 

SYMPATHETIC  NERVOUS  SYSTEM.    See  Autonomic. 

SYNAPSE.  The  contact  of  one  nerve  cell  with  another,  which  makes 
possible  the  conduction  of  a  nervous  impulse  from  cell  to  cell. 


GLOSSARY  453 

SYNAPSIS.     The  pairing  of  h^molp4ous  chromosomes  during  matu- 
ration of  the  germ  cells. 
SYNAPTIC   MATES.    Homologous   chromosomes   of  maternal  and 

paternal  origin  paired  in  synapsis. 
SYNGAMY.    The  union  of  gametes  to  form  a  zygote. 
SYNKARYON.    The  composite  nucleus  formed  by  the  union  of  the 

nuclei  of  two  gametes.     Male  and  female  pronuclei  united  to 

form  the  fertilization  nucleus.     See  Zygote. 
TAPIR.     A  large  herbivorous  Mammal,  having  short  stout  limbs  and 

flexible  proboscis  with  the  nostrils  near  the  end.      New  World 

species  are  brownish-black,  those  of  the  Old  World  are  black  and 

white. 

TAXONOMY.    The  science  of  classification. 
TELOPHASE.     Final  phase  of  mitosis  during  which  the  two  daughter 

nuclei  are  reformed  and    cytoplasmic   division   is   completed. 

See  Prophase. 
TESTIS.    The  definitive  male  reproductive   organ  in  which  the 

gametes  (sperm)  develop, 
THALLUS.     A  relatively  simple  plant  body,  not  differentiated  into 

root,  stem,  and  leaf ;  e  .g . ,  in  Seaweeds  and  other  multicellular  Algae 
THORAX.     The  anterior  chamber  of  the  coelom  in  Mammals,  con- 
taining lungs  and  heart.    The  middle  portion  of  the  body  in  the 

Arthropoda;  e.g.,  in  all  Insects.     In  the  Crayfish  the  head  and 

thorax  are  fused  to  form  the  cephalothorax. 
THYMUS.     A   glandular  structure   in   the   pharyngeal   region  of 

Vertebrates.    Disappears  during  early  life  in  Man.     Function 

unknown. 
THYROID.    A   glandular   structure   in   the   pharyngeal  region   of 

Vertebrates.     Supplies  an  important  hormone. 
TISSUE.    An  aggregation  of  similar  cells  for  the  performance  of  a 

certain  function.     See  Organ. 
TRACHEIDS.    Elongated  cells  which  form  water-conducting  vessels 

in  the  vascular  bundles  of  higher  plants. 
TRANSPIRATION.    The    exhalation    of    water   vapor,    particularly 

through  the  stomata  of  higher  plants. 
TRICHOCYSTS.     Minute  bodies,  arranged  in  the  outer  part  of  the 


454  APPENDIX 

ectoplasm  of  certain  Infusoria  (e.g.,  Paramecium),  each  of  which 
upon  proper  stimulation  is  transformed  into  a  thread-like  process 
protruding  from  the  cell  surface.  Apparently  defensive  struc- 
tures. 

TRIHYBRID.  The  progeny  of  parents  differing  in  regard  to  three 
given  characters. 

TRILOBITES.  Crustacea  dominant  during  the  early  Paleozoic  era. 
Extinct. 

TURGOR.  Outward  pressure  of  the  cell,  largely  due  to  the  absorp- 
tion of  water,  which  distends  the  cell  wall.  The  turgidity  of  the 
individual  cells  results  in  the  semi-rigid  position  of  many  plants. 
Wilting  results  from  a  lowering  of  the  turgidity  of  the  cells. 

TYPHLOSOLE.  A  median  dorsal  invagination  along  the  entire  length 
of  the  intestine  of  the  Earthworm.  Increases  the  area  of  the  di- 
gestive and  absorptive  surface. 

UMBILICAL  CORD.  A  Mammalian  structure,  commonly  known  as 
the  navel  cord,  by  which  the  embryo  is  attached  to  the  placenta. 
The  blood  vessels  from  the  embryo  to  the  placenta  pass  through 
it.  See  Placenta. 

UNGUICULATE.     Provided  with  claws. 

UNIFORMITARIAN  DOCTRINE.  An  interpretation  of  the  present  con- 
dition of  the  Earth  on  the  assumption  of  similarity  of  factors  at 
work  during  past  ages  and  to-day. 

UNIPARENTAL.  Derived  from  a  single  progenitor;  e.g.,  in  asexual 
reproduction.  See  Biparental. 

UNIT  CHARACTERS.  Characters  which  behave  more  or  less  as  units 
in  heredity. 

UREA.  Nitrogenous  waste  product  of  animal  metabolism.  Formed 
as  such  in  the  liver,  removed  from  the  blood  by  the  kidneys  and 
eliminated  from  the  body  chiefly  in  urine. 

URETER.  A  tube  carrying  urine  from  kidney  to  the  cloaca  or  to  the 
urinary  bladder. 

UROGENITAL.     Relating  to  the  urinary  and  reproductive  systems. 

UTERUS.  Lower  portion  of  the  oviduct  (or  oviducts)  modified  for 
the  retention  of  the  eggs  temporarily  (Frog)  or  until  develop- 
ment has  proceeded  a  considerable  way  and  'birth'  occurs  (Man) . 


GLOSSARY  455 

UTRICULUS.  The  posterior  sac  of  the  labyrinth  of  the  ear  into 
which  the  semicircular  canals  open. 

VASCULAR  BUNDLE.  Composite  of  xylem,  cambium,  phloem,  and 
bundle  sheath.  Except  for  the  cambium,  essentially  a  system 
of  tubes  for  conducting  water  and  food.  A  fibro-vascular 
bundle.  See  Stele. 

VASOMOTOR  NERVES.  Nerves  which  regulate  the  calibre  of  small 
arteries  by  bringing  about  relaxation  or  contraction  of  the 
muscular  layer  of  their  walls. 

VERMIFORM  APPENDIX.  Blind  outpocketing  of  the  large  intestine 
near  its  origin  from  the  small  intestine.  Vestigial  end  of  the 
caecum.  Found  only  in  Apes  and  Man. 

VERTEBRA.  One  of  the  series  of  elements  forming  the  backbone, 
or  vertebral  column. 

VERTEBRATE.     An  animal  with  a  backbone,  or  vertebral  column. 

VITALISM.  The  doctrine  which  attributes  at  least  some  of  the  phe- 
nomena of  life  to  an  interplay  of  matter  and  energy  which  tran- 
scends the  so-called  laws  operable  in  the  inorganic  world.  See 
Mechanism. 

VITAMINES.  Indispensable  accessory  food  substances  whose  im- 
portance has  but  recently  been  realized.  Chemical  composition 
is  as  yet  practically  unknown. 

WORKING  HYPOTHESIS.  A  basic  assumption  to  guide  the  study  of 
a  subject,  and  to  be  proved  or  disproved  by  facts  accumulated. 

X  CHROMOSOME.    The  'accessory'  or  'sex-chromosome.' 

XYLEM.     The  inner  woody  part  of  a  vascular  bundle. 

YEAST.  A  group  of  unicellular  colorless  plants  (Fungi)  which  are 
chiefly  responsible  for  alcoholic  fermentation. 

YOLK.  Food  material  stored  within  the  cytoplasm  of  an  egg.  See 
Metaplasm. 

ZOOGEOGRAPHY.  The  science  of  the  geographical  distribution  of 
animals. 

ZYGOTE.  The  composite  cell  formed  by  the  union  of  male  and 
female  gametes.  See  Synkaryon. 


INDEX 

[Figures  in  italics  designate  pages  on  which  illustrations  occur.] 


Abdomen,  130,  158 
Abdominal  cavity,  140 
Abdominal  pores,  207 
Abdominal  vein,  165 
Abiogenesis,  209,  210,  388 
Absorption,  87,  156,  158 
Accessory    chromosome     (see    X 

chromosome) 
Acoelomates,  121 
Acquired  character,  266,  297,  306, 

377,  408,  409 
Adam's  apple,  153,  356 
Adaptability,  individual,  339-344 
Adaptation,  11,  17,  18,  307-344, 

375;    functional,    308-313;     to 

living    environment,     330-339; 

physical  environment,  308-329; 

structural,  313-329 
Adaptive  radiation  of  Mammals, 

313-319 

Adaptive  variation,  378 
Adrenal  body,  152 
Adventitious  roots,  67 
Aerial  roots,  66,  67 
Afferent  nerve,  192 
Air  bladder,  148 
Air  spaces,  83,  84 
Alcoholism,  267 
Algae,  classification,  413 
Alimentary  canal,   121,  123,  131, 

137,  148-156;   derivatives,  160 
Alimentary  system,  116 
Allelomorphs,  276,  280,  282,  288 
Alligator,  brain,  189 


Alternation  of  generations,  jj4? 
100-114,  218-220,  229 

Alternative  inheritance  (see  In- 
heritance) 

Amines,  36,  87 

Amino  acids,  13,  42,  158 

Amoeba,  9,  19,  116,  340 

Amphibian,  117,  136,  149,  189  (see 
Frog) 

Amphioxus,  146,  259,  415;  devel- 
opment, 257]  egg,  256 

Anabolism,  16 

Anaerobe,  311 

Analogous  structures,  63,  130,  199 

Anaphase,  225 

Anatomy,  4\  comparative,  132, 
351-356;  history,  393,  394 

Ancestral  inheritance,  law  of,  270 

Animal  body,  115-153;  versus 
plant  body,  115 

Animal,  chief  groups,  116,  117; 
circulation,  161-174;  classifica- 
tion, 116,  117,  414-416;  colora- 
tion, 319-324;  coordination, 
181-202;  excretion,  175-180; 
metabolism,  39-43;  nutrition, 
154-160;  reproduction,  203- 
208;  respiration,  161-174;  ses- 
sile, 115;  unicellular,  39 

Annual  plant,  66 

Antenna,  131,  132]  cleaner,  326; 
comb,  326 

Antennule,  131,  132 

Anther,  108,  110,  112 

Antheridium,  101,  104 


457 


458 


INDEX 


Antibody,  338 

Antitoxin,  338 

Antlers,  206 

Ants,    instincts,    343;     associated 

with  Aphids,  333 
Anus,  121,  153 
Aorta     (see   Dorsal  and  Ventral 

aorta) 

Aortic  arches,  171 
Aphids  and  Ants,  333 
Apis  (see  Bee) 
Aqueous  humor,  201 
Arabian  scientists,  383 
Archaeopteryx  and  Pigeon,  360 
Archegonium,  101,  104 
Aristotle,  2,  15,  209,  379,  380,  383, 

390,  394,  398,  401,  406 
Arterial  system,  166 
Arteries,     163-165;      pulmonary, 

150,  151,  152 
Arterioles,  163 
Arthropoda,    129;     classification, 

415;  structure  of  primitive,  130 
Artificial  parthenogenesis,  249 
Ash,  80 
Asparagine,  36 
Asparagus,  70 
Aspidium,  103,  104 
Associations,  communal,  331 
Aster,  225 
Atavism,  269 
Auditory  capsule,  145 
Auditory  nerve,  197,  198 
Aurelius,  Marcus,  44,  382 
Auricle,  163,  172  (see  Circulation) 
Autonomic  nervous  system,   186, 

191,  192 
Azalea,  75 

B 

Babylonian  science,  379 
Bacillus  tetani,  311 
Bacon,  R.,  386 


Bacteria,  44-53;  discovery,  388; 
chief  types,  45 ',  denitrifying,  49 ; 
as  food,  42;  nitrate,  48;  nitro- 
gen-fixing, 49,  333;  nutrition, 
50;  reproduction,  46;  sulfur, 
309;  types  of  flagellation,  46 

von  Baer,  1,  402 

Balanced  aquarium,  53 

Barberry,  72 

Bark,  88 

Barley,  80 

Barnacle,  115 

de  Bary,  7 

Bat,  318;   wing  skeleton,  352 

Beagle,  voyage  of,  369 

Bean,  66;  inheritance  in,  300; 
section  of  stem,  81 

Beaver,  349 

Bee,  325,  415;  head,  326;  in- 
stincts, 343;  legs,  324-329; 
parthenogenesis,  249;  pollina- 
tion by,  330 

Bee-fly,  323 

Beggiotoa,  309 

Bernard,  91 

Bibliography,  417-428 

Biennial  plant,  67 

Bilateral  symmetry,  124 

Bile  duct,  137,  148,  151,  152,  155 

Binomial  nomenclature,  391 

Biochemistry,  5 

Biogenesis,  210,  388 

Biogenetic  law,  364,  403 

Biological  sciences,  5 

Biology,  1;  divisions  of,  4]  his- 
tory, 379-411;  and  medicine, 
381;  scope  of,  1-5 

Biophysics,  5 

Biparental  inheritance,  251  (see 
Inheritance) 

Biramous  appendage,  132 

Bird,  117,  136;  brain,  189;  circu- 
lation, 165;  dissection  of,  151; 


INDEX 


459 


egg,  238}  embryo,  366}  versus 
Reptile,  359;  skeleton  of  wing, 
352 

Birth,  208 

de  Blainville,  15 

Blastocoel,  57 

Blastoderm,  238 

Blastopore,  57,  58,  127 

Blastostyle,  218 

Blastula~  57,  58,  126,  252 

Blending  inheritance,  268,  283,  286 

Blood,  163;  capillary  circulation 
discovered,  389;  circulation 
demonstrated,  385;  corpuscles, 
163,  338;  pressure,  173;  rate 
of  flow,  172;  relationships,  367; 
specific  differences,  367;  trans- 
fusion, 367 

Body,  animal,  115-153;  plant, 
61-90 

Body  plan  of  Earthworm,  122 

Body  plan  of  Vertebrates,  136-138 

Body  temperature,  174,  176,  312 

Bone  (see  Skeleton) 

Borelli,  395 

Botany,  3,  4 

Brain,  134,  137,  148-152}  evolu- 
tion of,  365;  human,  153}  ven- 
tricles, 188 

Branchial  arteries,  165,  166 

Branchial  clefts,  164  (see  Gill  slits) 

Bryales,  101,  414 

Bryophyta,  101,  112,  413 

Bud,  81,  82}  winter,  72 

Budding,  113,  119,  213 

Buffon,  407 

Bulb,  70 

Buttress  root,  67 


Cactus,  70 

Calciferous  gland,  123 
Calyx,  75,  107 


Cambarus  (see  Crayfish) 
Cambium,  60,  76,  81 
Camel,  evolution  of,  363 
Cameron,  E.  H.,  344 
Capillaries,    163,    173;    of  lungs, 

165}  network,  159,  389 
Carbohydrates,  13,  14,  35,  42, 157 
Carbon  cycle,  48 
Carnivora,  349 
Carotid  artery,  164,  165,  171 
Carpal,  144,  145 
Carpel,  75,  107,  108,  112 
Cartilage,  25,  140 
Castor,  349 
Catalyzer,  14 

Cat,  brain,  189;  skeleton,  145 
Catocala,  320 

Caudal  artery  and  vein,  164 
Cell,  21;  ciliated,  &J;  defined,  23; 

diagram  of,  26}   discovery,  3J7; 

division,  29,  225,  227;  doctrine, 

399;    epithelial,   59}    forms  of. 

23-29;    nerve,  25}    origin,   28; 

plant,  generalized,  77;   sap,  26, 

80,  84;    theory,  242;    wall,  27, 

77 

Cell  cycle,  214,  215 
Cellulose,  13,  31 
Cenozoic  era,  358 
Central  cylinder,  76,  79 
Central  nervous  system,  186  (see 

Nervous  system) 
Central  spindle,  226 
Centrosome,  26,  27,  225 
Centrum,  141,  143,  145 
Cephalothorax,  131 
Cerebellum,  153,  187,  189 
Cerebral  ganglion,  128,  131,  134 
Cerebral   hemispheres,    153,    187, 

188,  189 
Chameleon,  321 
Characters,    acquired,    266,    297 

306,  377,  408,  409;   alternative, 


460 


INDEX 


405;  dominant,  272;  linked, 
268,  291,  293;  recessive,  272; 
unit,  262,  280,  282,  286 

Cheliped,  131 

Chemical  coordination,  181-183 

Chemistry,  origin  of,  395 

Chemosynthesis,  50 

Chemotaxis,  240 

Cheshire,  F.  R.,  328 

Chipmunk,  349 

Chlorenchyma,  77,  83 

Chlprophyll,  22 ',  chemical  compo- 
sition of,  35 

Chlorbplastid,  35,  84 

Choloepus,  317 

Chordate,  146,  415 

Choroid,  201 

Chromatin,  28;  knot,  26',  net- 
work, 26 

Chromomere,  234 

Chromosome,  225,  242,  296',  ac- 
cessory, 292;  combinations, 
290;  diploid  number,  235,  289; 
distribution,  236;  division,  226; 
duplex  groups,  231,  235,  289; 
haploid  number,  235,  289; 
homologous,  234,  235,  287,  289; 
individuality,  227;  in  Man,  237, 
291,  292;  maternal,  234,  235, 
287;  pairs,  236;  paternal,  234, 
235,  287;  reduction,  228;  segre- 
gation, 290;  sex,  292,  295,  377; 
simplex  groups,  231,  235,  -289; 
synapsis,  285]  X,  282-295,  377 

Chromosome -cycle,  233-237;  in 
animals,  229,  289;  diagram  of, 
235;  in  plants,  229,  288 

Cilium,  19,  25,  40 

Circulation,  in  animals,  116,  122, 
131,  161-174;  in  Flowering 
Plants,  85-88 

Class,  350 

Classification,  animals,   116,   117, 


414-416;  Algae,  413;  Arthro- 
poda,  415;  Eutherian  Mam- 
mals, 416;  Ferns,  414;  Flower- 
ing Plants,  414;  Fungi,  413; 
history  of,  390;  Mammals,  416; 
Mosses,  413,  414;  plants,  413, 
414;  Protozoa,  414;  Verte- 
brates, 416 

Clavicle,  141,  144 

Claws,  138 

Cleavage,  55 

Cloaca,  137,  149,  156 

Clover,  80 

Clustered  roots,  65 

Coccyx,  153,  355 

Cochlea,  196 

Coelenterata,  118,  414  (see  Hydra 
and  Obelia) 

Coeliac  artery,  164 

Coelom,  120,  137,  140,  IBS 

Coelomate,  121 

Coelomic  fluid,  162,  177 

Collar  bone,  144 

Colloidal,  8 

Colony,  213 

Coloration,  animal,  319-324 

Color-blindness,  inheritance  of, 
294,  295 

Colorless  plants,  43-53 

Columba  (see  Pigeon) 

Combinations,  268,  269,  302,  377 

Comparative  anatomy  (see  Anat- 
omy) 

Conduction,  18,  183 

Condylarthra,  361 

Conjugation,  41,  214,  244,  250; 
diagram  of,  245 

Conjunctiva,  201 

Connective  tissue,  142 

Conservation  of  energy,  396 

Contractile  vacuole,  9,  40 

Contractility,  33 

Conus  arteries  us,  165,  166 


INDEX 


461 


Coordination  in  animals,  181-202; 
chemical,  181-183;  by  nervous 
system,  183-193 

Copulatory  organs,  132,  152,  205 

Coracoid,  141,  144 

Corm,  70 

Cornea,  201 

Corolla,  75,  107 

Correlation  of  structure  and  func- 
tion, 393 

Cortex,  60,  77,  79,  81 

Cortical  system,  76 

Cotyledons,  66,  87 

Cranial  nerves,  188,  190 

Cranium,  148-153 

Crayfish,  129-135,  169,  415;  ap- 
pendages, 132,  353;  circulatory 
system,  131;  copulatory  organs, 
132,  205;  dissection  of,  131; 
feeding  instincts,  342 

Cretin,  182 

Crop,  of  Earthworm,  123;  of  Bird, 
151 

Crossing-over,  262;  mechanism 
296 

Crura  cerebri,  188 

Crustacea,  117,  415  (see  Crayfish) 

Cursorial,  314 

Curve  of  probability,  299-305 

Cutaneous  senses,  195 

Cuvier,  3£>£-394 

Cyclostomes,  179,  416 

Cynthia,  254,  259,  415 

Cytology,  4,  287,  403 

Cytoplasm,  24-27 

Cytoplasmic  differentiation,  257, 
258,  259;  organization,  254; 
zones,  259 

Cytotoxin,  339 

D 

Dahlia,  65 
Daltonism,  294 
Dandelion,  65 


Darwin,  C.,  262,271,  299,369,  374- 
376,  378,  410,  411,  frontispiece 

DarwiA,  E.,  209,  408 

Darwinism,  present  status,  378 

Dead-leaf  Butterfly,  321 

Dentalium,  254,  259,  415;   devel- 
opment, 257 

Dermal  system,  76 

Dermis,  human,  139 

Descent    with    modification    (see 
Evolution) 

Dextrin,  13 

Diapheromera,  322 

Diaphragm,  140,  153,  155 

Dicotyledons,  82,  414 

Diencephalon,  187,  188 

Digestion,  42,  87,  157-160,  395 

Digitigrade,  316 

Digits,  144,  361 

Dihybrid,  276-280 

Dinosaur,  116,  416 

Dioscorides,  382 

Diploid  number,  229,  234,  235 

Disease,  inheritance  of,  267 

Distribution,     368-372;      discon- 
tinuous, 368 

Division  of  labor,   physiological, 
28,  57,  117,  324 

Dodder,  68 

Dogfish,  164,  416 

Dominance,    272,    282,    286;     in- 
complete, 283;  lack  of,  283 

Dorsal^orto,  137/150,  151,  166 

Dorsal  roo^  192  / 

Ductless  glands,  159 

Ducts,  of  glands,   151,  155,  158; 
in  plant  stem,  86 

Dujardin,  7 

Duplex  group,  234 

E 

Ear,  196-198 
Earth,  age  of,  357 


462 


INDEX 


Earthworm,  121-129,  159,  162, 
415;  body  plan,  122;  circula- 
tory system,  122;  dissection  of, 
123;  excretion,  177;  feeding 
instinct,  342;  nerve  cells,  194; 
nerve  cord,  184;  reflex  arc,  184; 
regeneration  and  grafting,  222; 
reproductive  organs,  204;  sen- 
sory and  motor  neurons,  184; 
transverse  section,  124 

Echinoderm,  259,  415  (see  Sea 
Urchin) 

Ecology,  4,  368 

Ectoderm,  22,  57,  119,  126,  185, 
215 

Ectoplasm,  27,  40 

Education  (see  Man) 

Efferent  nerve  (see  Nerve) 

Egg,  101,  104,  106,  112,  236,  253; 
of  Cat,  25;  changes  at  fertiliza- 
tion, 241;  human,  239;  Mam- 
mal, 238,  239,  402;  membrane, 
241;  organization  of,  236,  254, 
255 

Egyptian  science,  379 

Elements,  cycle  of,  43,  46-50,  309 

Elephants,  evolution  of,  371;  geo- 
logical and  geographical  dis- 
tribution, 370 

Elodea,  82 

Embryo,  112;  Fish,  Bird,  Man, 
205,  366 

Embryology,  4,  252,  364-366; 
comparative,  402;  of  Earth- 
worm, 125-129;  experimental, 
255,  403;  history  of,  401-403 

Embryo  sac,  108,  112 

Empedocles,  406 

Emulsoid,  8 

Encyclopaedists,  384 

Endocrine,  312;  glands,  159  (see 
Thyroid  and  Chemical  coordi- 
nation) 


Endocrinology,  182 

Endoderm,  22,  57,  119,  126,  215 

Endomixis,  41,  247,  250;  nuclear 
changes,  248 

Endoplasm,  27,  40 

Endopodite,  132 

Endoskeleton,  140  (see  Skeleton) 

Endosperm,  110,  111,  112 

Energy,  conservation  of,  396; 
from  sun,  37,  38;  transforma- 
tion of,  15,  38  (see  Kinetic  and 
Potential  energy) 

Enteric  cavity,  57,  119 

Enteron,  22 

Environment,  fitness  of,  307;  in- 
fluence of,  266,  267,  290-298, 
409  (see  Adaptation) 

Enzymes,  14,  37,  156,  158,  310 

Eohippus,  361,  362 

Epencephalon,  188  (see  Cerebel- 
lum) 

Epidermis,  22;  60,  77,  79,  81,  139 

Epigenesis,  253,  257,  259,  401 

Epithelium,  156 

Epochs  in  biological  history,  379- 
411 

Equation  division,  231,  232 

Equatorial  plate,  225,  226,  242 

Equipotent,  258 

Equus  (see  Horse) 

Eristalis,  323 

Eugenics,  297 

Eustachian  tube,  149,  150,  198, 
356 

Euthenics,  297 

Evaporation,  89 

Evolution,  4,  129,  185,  251,  262, 
267,  345-378;  of  Camel,  363; 
of  Elephant,  371;  evidences 
of,  347-372;  factors  of,  372- 
378;  and  heredity,  376;  of 
Horse,  361,  362;  history  of, 
406-411 


INDEX 


463 


Exconjugant,  245 

Excretion,  16;  in  animals,  13, 
175-180 

Excretory  system,  116;  evolu- 
tion of,  179,  365 

Exopodite,  132 

Exoskeleton,  140,  146 

Experimental  biology,  252 

External  receptor,  193 

Extracted  dominant,  274 

Extracted  recessive,  274 

Eye,  of  Arthropod,  199;  of  Cuttle- 
fish, 199;  development  of,  200; 
human,  diagram  of,  201 ;  Inver- 
tebrate, 199;  optic  stalk,  199; 
origin  of,  198-200;  rods  and 
cones,  201;  Vertebrates  vs.  In- 
vertebrates, 200 


Fabricius,  401 

Factors,  multiple,  286 

Faeces,  156,  175 

Fallopian  tubes,  205  (see  Oviduct) 

Fats,  13,  14,  42,  157,  158 

Fat  body,  149 

Feather,  138 

Fermentation,  47;  alcoholic,  310 

Ferns,  classification  of,  414;  fer- 
tilization in,  240;  life  history, 
103,  104,  107 

Fertilization,  34,  113,  114,  214, 
231,  235,  237-242,  245,  249-251; 
Protista,  243-248;  significance 
of,  242-251 

Fibrous  root,  65 

Fibula,  144 

Fig,  67 

•Filial  regression,  law  of ,  270,  301, 
303 

Fins,  138,  141 

Fish,  117,  136;  brain,  189;  circu- 
lation, 164,  165,  171;  classifi- 


cation, 117,  135,  416;  dissec- 
tion of,  148;  embryo,  866;  res- 
piratory current,  170;  skeleton, 
141 

Fission,  binary,  212;  multiple,  212 

Fixity  of  species,  346 

Flagellum,  33 

Flatworm,  414;  fission,  217;  re- 
generation, 223 

Flax,  110 

Flexures,  cranial,  188 

Floral  parts,  75,  107,  108 

Flower,  107,  112;  staminate,  355; 
vertical  section,  110 

Flowering  plants,  61,  105;  classi- 
fication, 414;  life  history, 
107-114;  physiology,  84-90; 
structure,  65-84 

Fluctuations,  300,  302 

'Flying  Lemur,'  318 

Foetal  membranes,  205 

Food,  157,  158,  308-311;  of  ani- 
mal and  green  plant  contrasted, 
42;  stuffs,  14,  42;  utilization, 
Flowering  Plants,  89,  90 

Fore-brain,  186,  187,  188,  189 

Four-o'clock,  283,  284 

Fovea  centralis,  201 

Fragmentation,  113 

Frequency  curve,  299-305 

Frog,  brain,  189;  circulation,  165; 
dissection  of,  149;  section  of  in- 
testine, 59  (see  Amphibian) 

Frond,  39,  103,  104 

Fructose,  13,  36          ^ 

Fruit,  66,  110,  112 

Fucus,  62 

Fungi,  classification  of,  413  (see 
Bacteria) 

G 

Galen,  382,  394,  396 
Galeopithecus,  318 


464 


INDEX 


Galileo,  387 

Gallapagos  Islands,  369 

Gallbladder,  148,  149,  155 

Galton,  301,  303,  404 

Gallon's  Laws,  269-271,  303 

Gamete,  54,  112,  228-230,  236; 
formation,  94-96;  evolution  of, 
237  (see  Egg  and  Sperm) 

Gametophyte,  64,  101,  105;  fe- 
male, 106,  110}  male,  106,  110 

Ganglion,  133,  185,  191 

Ganong,  W.  F.,  61,  111 

Gastric  juice,  157,  158 

Gastric  vacuole,  40 

Gastroliths,  131 

Gastrula,  57,  58,  156,  252 

Geddes,  P.,  307 

Gel,  8 

Gene,  236,  280,  286,  377;  altera- 
tion of,  298;  modifying,  304; 
multiple,  285,  286;  segregation 
of,  290 

Genetics,  4,  261-306;  history  of, 
403-406 

Genital  duct,  137 

Genotype,  275,  277,  279,  281 

Genus,  in  classification,  349 

Geological  time  table,  356 

Geomelrid  Moth,  larva,  322 

Germ  cells,  215,  229;  origin,  223- 
242;  primordial,  224 

Germ  layer,  58,  128;  theory,  403 

Germ  plasm,  222,  265,  377,  404 

Germinal  continuity,  216,  222, 
264,  265,  377,  404 

Gesner,  384 

Gill,  175;  pouches,  155,  169;  slits, 
137,  147,  164,  366 

Giraffe,  266 

Gizzard,  of  Earthworm,  123]  of 
Bird,  151 

Gland,  Cowper's,  152;  diagram 
of,  159}  ductless,  159]  endo- 


crine, 159;  oil,  151}  prostate, 
152}  salivary,  159;  sebaceous, 
139]  sweat,  139,  176;  thymus, 
150,  152,  155,  159;  thyroid, 
152,  155,  159,  182;  unicellular, 
59,  158 

Glossary,  429-455 

Glottis,  149,  150,  151 

Glucose,  13,  36 

Glycerine,  14 

Goethe,  410 

Goitre,  182 

Gonad,  137,  164,  179,  203  (see 
Ovary  and  Testis) 

Gorilla,  skeleton  of,  354 

Grafting,  221 

Grape,  70 

Grape  sugar,  87 

Grass,  65,  70 

Greek  natural  philosophers,  345- 
406 

Greek  science,  2,  379-382 

Green  gland,  131 

Green  plants,  30-38 

Gregarious  animals,  331 

Grew,  388 

Growing  point,  76,  79,  81,  82,  87 

Growth,  by  accretion,  16;  by 
intussusception,  11,  16,  19 

Growth  zone,  78 

Guard  cells,  22,  83 

Gulfweed,  63 

Gullet,  40 

Gymnura,  315 

H 

Hair,  139]    character,  inheritance 

of,  278 
Hales,  397 
Haller,  395 

Haploid  number,  229,  234,  $35 
Harvey,  161,  243,  385,  386,  389, 

394,  401 


INDEX 


465 


Haustoria,  68 

Hay  infusion  microcosm,  50-51 

Head,  130,  190;  of  Honey  Bee,  326 

Heart,  131, 137, 148-151, 153,  163, 
165,  172;  evolution  of,  365; 
work  of,  172 

Heat,  animal,  15  (see Temperature) 

Hedgehog,  313,  416 

Hematochrome,  35 

Henderson,  L.  J.,  308 

Hen's  egg,  238,  253 

Hepatic  artery  and  vein,  164,  165, 
167 

Hepatic  portal  system,  150,  165, 
167,  168 

Herbalists,  385 

Herbals,  383 

Heredity,  251;  and  evolution, 
376;  'social,'  297  (see  Inherit- 
ance) 

Heritage  of  the  individual,  261- 
306 

Hermaphrodite,  204 

Hertwig,  O.,  21 

Heterospory,  106 

Heterozygote,  277,  279,  281 

Heterozygous,  276 

Hickory,  72 

Hind-brain,  186  (see  Brain) 

Hippocrates,  381 

Histology,  4;  history  of,  398-400; 
plant,  75-84  (see  Tissue) 

History,  of  biology,  379-411;  com- 
parative anatomy,  392-394; 
embryology,  401-403;  genetics, 
403-406;  histology,  398-400; 
organic  evolution,  406-411; 
physiology,  394-398;  taxonomy, 
390-392 

Holdfast,  63 

Homologous  chromosome  (see 
Chromosome) 

Homologous  organs,  130,  852 


Homology,  serial,  353 

Homothermal,  174,  176,  312 

Homozygote,  277,  279,  281 

Homozygous,  276,  302 

Honey  Bee  (see  Bee) 

Hoofs,  138 

Hooke,  387 

Hormone,  181,  206  (see  Endo- 
crine) 

Horns,  138 

Horse  Chestnut  bud,  72 

Horse,  evolution,  361,  362;  skele- 
ton of  leg,  352 

Host,  334 

Human,  body,  chemical  composi- 
tion, 11;  median  section,  153; 
ear,  198;  egg,  239;  eye,  201; 
kidney,  180;  skeleton,  354; 
sperm,  239  (see  Man) 

Humerus,  144 

Hutton,  410 

Huxley,  30,  54,211,  261,  345,  356, 
393,  394 

Hybrid,  272,  306  (see  Heterozy- 
gote) 

Hydra,  118-121,  157,  169,  194, 
219,  332,  414;  asexual  reproduc- 
tion, 217;  discovery  of,  388; 
feeding  instinct,  342;  longitudi- 
nal section,  119;  nerve  cell,  183; 
receptor-effector  system,  184; 
regeneration  and  grafting,  221; 
reproductive  organs,  204;  trans- 
verse section,  22,  120 

Hydranth,  218;  comparison  with 
medusa,  219 

Hydrochloric  acid,  157 

Hydroid,  115,  117,  145;  colony, 
218;  life  history,  218 

I 

Iliac  artery  and  vein,  164 
Ilium,  144,  145,  354 


466 


INDEX 


Immunity,  338,  339 

Incus,  198 

Indian  Corn,  67,  263 

Individual,  adaptability,  339-344; 
origin  of,  209-260;  heritage  of, 
261-306 

Infundibulum,  187-189 

Ingenhousz,  398 

Inheritance,  261,  403;  alterna- 
tive, 268,  405;  blending,  268, 
283,  286;  of  human  hair  char- 
acters, 278;  mosaic,  268;  sex- 
linked,  268;  of  size  in  Peas,  278 

Insects,  117,  415;  traps,  73 

Instincts,  342 

Integumentary  system,  116 

Intercellular  digestion,  156 

Internal  receptor,  193 

Intestine,  123,  131,  137,  148,  156; 
nerve  supply,  186;  section  of, 
59 

Intracellular  digestion,  156 

Invertebrates,  117,  414,  415 

Invertebrate  eye,  199,  200 

Iris,  201 

Irritability,  18,  181,  183 

Ischium,  144,  145,  354 

Islands,  continental,  369;  coral, 
372 

Island  faunas  and  floras,  369 

Ivy,  67 


Jaws,  143 

Jennings,  H.  S.,  342 
Johannsen,  303 
Jugular  vein,  164,  165 
Jurassic  period,  358 

K 

Kallima,  321 
Karyolymph,  27 
Karyosome,  26,  28 


Kata  holism,  16 

Katydid,  319 

Kelley,  H.  A.,  148-152 

Kelp,  63 

Kidney,  137,  153,  175,  180 

Kinetic  energy,  6,  35,  42 


Labyrinth,  196 

Lacteals,  168 

Lagena,  197 

Lamarck,  3,  39,  267,  374,  393,  409 

Lamina,  71 

Lamprey,  416;  egg  and  sperm,  236 

Laplace,  15 

Larynx,  153,  356 

Latent  character,  268 

Lateral  line  organs,  195 

Lavoisier,  15,  396 

Law  of  probability,  299-30-5 

Leaf,  65,  71-75,  82-84,  112;  air 
spaces,  83]  base,  71;  develop- 
ment, 82;  epidermis,  83;  pali- 
sade layer,  83;  section  of,  83; 
vein,  83;  vertical  section,  22 

Leeuwenhoek,  387,  388 

Legs  of  Bee,  327 

Lens,  201 

Lichen,  332 

Liebig,  398 

Life,  6,  19;  definition,  15;  origin, 
28,  209;  physical  basis,  6,  400; 
'triangle'  of,  298;  web  of,  330 

Limb,  pentadactyl,  144  (see  Skele- 
ton) 

Linin,  26,  27 

Linkage,  293-296 

Linnaeus,  346,  390-392 

Linville,  H.  R.,  148-152 

Liver,  137,  148,  149,  156,  168,  175 

Living  matter,  adaptation,  17-18; 
characteristics  of,  10-20;  chemi- 


INDEX 


467 


cal  composition,  11-14;    organ- 
ization, 18,  19 

Lizard,  dissection  of,  150 

Lockjaw,  311 

Loiseleuria,  75 

Lumbricus  (see  Earthworm) 

Lungs,  150,  153,  161,  175 

Lyell,  410 

Lymph,  156,  163,  168,  173 


M 

Macronucleus,  40,  41,  245,  248 

Malaria,  336 

Malarial  Parasite,  life  history,  335 

Malleus,  198 

Malpighi,  388,  389,  401 

Mammal,  117,  136;  adaptive  radi- 
ation of,  313-319;  brain,  189; 
circulation,  165;  copulatory 
organs,  152',  classification  of, 
416;  dissection  of,  152]  egg  of, 
238,  239;  Eutherian,  313,  416 

Mammary  glands,  206 

Man,  body  temperature,  312; 
digestion,  158;  education,  297, 
344;  embryo,  366;  inheritance 
in,  297;  skeleton,  354;  skeleton 
of  arm,  352  (see  Human) 

Mandible,  132,  145 

Marsilia,  106 

Mastodon,  370 

Mathews,  A.  P.,  181,  344 

Matter,  non-living  and  living  as- 
sociated, 7 

Maturation,  288  (see  Oogenesis 
and  Spermatogenesis) 

Maxilla,  132,  145 

Maxilliped,  131,  132 

Mechanism,  257 

Medicine  and  biology,  381 

Medieval  science,  382,  383 

Medulla,  153,  187, 189  (see  Brain) 


Medusa,  218,  compared  with  hy- 

dranth,  219 

Megasporangium,  108,  112 
Megaspore,  106,  107,  110,  112 
Megasporophyll,  106,  108,  112 
Mendel,  271-274,  404,  405 
Mendelism,  271-306;  general  prin- 
ciples, 280-282;    in  Man,  278; 

mechanism    of,    287-296,    405; 

laws    rediscovered,  405;    ratio, 

272  (see  Monohybrid,  Dihybrid, 

and  Trihybrid) 
Meristem,  77,  78,  79 
Mesenteric  artery,  164 
Mesentery,  137,  149 
Mesoderm,  57,  126,  215;    bands, 

126;  somatic,  126;   splanchnic, 

186 

Mesogloea,  118,  120,  219 
Mesohippus,  361,  368 
Mesonephric  duct,  148,  179 
Mesoriephros,  137,  148,  179 
Mesozoic  era,  358 
MeiahoJism,  15^1^.175,  181,  307; 

animals,  39-43;    Bacteria,  44- 

53;      colorless    plants,     44-53; 

green  plants,  34-38 
Metagenesis,  220 
Metamere,  121-183,  190 
Metamerism,  121,  124,  127,  130, 

191 

Metanephric  duct,  179 
Metanephros,  150,  151,  152,  153, 

179 

Metaphase,  225 
Metaphyta,  54 
Metaplasm,  26 
Metazoa,  54,  116 
Metencephalon  (see  Medulla) 
Microcentrum,  319 
Micronucleus,  40,  41,  245,  248 
Microscope,  7;   invention  of,  386, 

387 


468 


INDEX 


Microscopists,  386-389 
Microsporangium,  108,  110,  112 
Microspore,  106,  107,  110,  112 
Microsporophyll,  106,  112 
Mid-brain,  186  (see  Brain) 
Millipede,  129,  415 
Mimicry,  protective,  323 
Mirabilis,  284 
Mistletoe,  68 
Mitosis,     29,     224-228;     typical 

stages  in,  225 
Mitral  valve,  165,  172 
Modifications,  265-267,  302,  304, 

444 

von  Mohl,  7 
Mole,  317 
Mollusc,   117,  415;    development 

of,  255;  nerve  cells,  194 
Monads,  52 
Monographers,  385 
Monohybrid,  272-276 
Morphogenesis,  403 
Morphology,  3 
Mosaic  inheritance,  268 
Mosquito,  336 
Moss,     105;     classification,    413, 

414;      fertilization,     240;      life 

history,  100-103 
Motor  nerve,  191 
Mouth,  41,  119,  121 
Movement,  amoeboid,  19;  ciliary, 

19;  power  of,  19 
Mulatto,  283-285;  recombination 

square,  285 
Muller,  396 
Muscles,  18,  138,  139;    cells,  59, 

183;  involuntary,  139;  smooth, 

25;     striated,    25;     voluntary, 

139;   of  eye,  201 
Muscular  system,  116 
Mutations,  269,  298,  306,  377 
Mycelium,  332 
Myotome,  139 


Myrsiphyllum,  71 
Myxedema,  182 

N 

Nails,  138 
Nares  (see  Nostrils) 
Natural  history,  2 
Natural  philosophy,  3 
Natural  selection,   306,   374-376, 

378,  411 

Nature  versus  nurture,  296-299 
Neo-Mendelism,  282-306 
Nephridium,  122,  123,  124,  181, 

177,  207 

Nephrostome,  124,  177,  208 
Nerve,  191;  afferent  and  efferent, 
192;     auditory,    197;     cranial, 
152,   188;    motor,    191;    optic, 
152;  sensory,  191 ;  sensory  end- 
ing, 139;  spinal,  188,  190;  tem- 
perature,    312;    trophic,     312; 
vagus,  190;  vasomotor,  174,  312 
Nerve  cells,  differentiation,  185 
Nerve  cord,  123,  134,  190 
Nerve  fibers,  185 
Nerve  net,  184,  186 
Nerve  plexus,  186,  190,  191 
Nervous  impulse,  191,  193,  194 
Nervous  system,  116,  133;  coordi- 
nation by,   183-193;    Crayfish, 
134;  Earthworm,  134;  Frog,  190 
Neural  arch,  137,  143 
Neural  canal,  137,  143,  153 
Neural  groove,  186 
Neural  tube,  186,  187,  190 
Neuro-muscular  mechanism,  183 
Neuron,  184-186,  193,  194 
Nictitating  membrane,  355 
Nitrates,  36,  43 
Nitrogen  cycle,  49 
Nitrogen-fixing  Bacteria,  49,  333 
Nitrogenous  wastes,  43 
Nomenclature,  Binomial,  391 


INDEX 


469 


Nostrils,  149,  152,  153 
Notochord,  137,  142,  146 
Nucleolus,  26 
Nucleus,  9,  26,  27,  28,  77;  during 

conjugation,  245;   during  endo- 

mixis,  248 

Nurture  versus  nature,  296-299 
Nutrition  of  animals,  154-160 
Nutritional  chain,  330 


Obelia,  414;  life  history,  218 

Oedogonium,  97,  98 

Oesophagus,  123,  131,  148-155 

Oil,  14;  gland,  151 

Olfactory,  bud,  196;  lobe,  IJfr, 
187,  189,  190;  pouches,  196; 
sense,  196 

Oligocene  period,  358 

Onion  leaf,  72 

Onoclea,  74 

Ontogeny,  364  (see  Embryology) 

Oocytes,  primary  and  secondary, 
231 

Oogenesis,  231-233 

Oogonium,  224,  232 

Operculum,  141 

Opsonin,  338 

Optic  capsule,  141,  145;  cup,  199; 
lobes,  188,  189',  nerve,  152, 
190]  stalk,  199 

Order,  in  classification,  349 

Organ,  59;  organ-forming  sub- 
stances, 256,  259 

Organic  evolution,  262  (see  Evolu- 
tion) 

Organisms,  adaptation  of,  307- 
344;  colonial,  55,  56;  micro- 
cosm, 20;  structure  of  multi- 
cellular,  54-60 

Organization,  11,  18 

Organ  systems,  60,  116 

Origin  of  the  germ  cells,  223-242 


Origin  of  the  individual,  209-260 

Origin  of  species,  345-378,  410, 411 

Osmosis,  86,  88 

Osteology,  140  (see  Skeleton) 

Ovary,  119,  123,  148,  203 

Oviduct,  128,  179,  205,  208 

Ovule,  110,  112 

Ovule  case,  108 

Owen,  394 

Ox,  skeleton  of  leg,  352 

Oyster,  135,  415 


Pain,  sense  of,  195 

Paleontology,  4,  356-364,  393 

Palm,  67 

Pancreas,  149,  152,  155,  158 

Pancreatic  duct,  151,  155 

Paramecium,  39,  52,  116,  157,  169; 
aurelia,  40,  244;  behavior,  340, 
341;  calkinsi,  40;  caudatum, 
40;  conjugation,  41,  245;  con- 
tractile vacuole,  40;  digestion, 
42;  division,  41  i  ectoplasm,  40; 
endomixis,  41,  248;  endoplasm, 
40;  excretion,  43;  focd  taking, 
42;  gastric  vacuole,  40;  gullet, 
40;  heredity  in,  264;  irrita- 
bility, 194;  macronucleus,  40, 
41, 245, 248;  metabolism,  41-43; 
micro-nuclei,  40,  41,  245,  248; 
mouth,  40,  41;  neuromotor  ap- 
paratus, 40;  peristome,  40; 
power  of  reproduction,  375;  re- 
production, 212,  246;  respira- 
tion, 43;  species,  40,  414;  struc- 
ture and  life  history,  39-41; 
trichocysts,  40 

Parasitism,  68,  220,  334-338 

Parencephala,  188  (see  Cerebral 
hemispheres) 

Parthenogenesis,  243,  249 

Pasteur,  210 


470 


INDEX 


Patella,  145 

Peas,  inheritance  in,  272-282 

Pectoral  girdle,  141,  144,  14$, 
354 

Peduncle,  107 

Pelvic  bones,  317,  144,  153 

Perca,  141,  148 

Perch,  dissection,  148 

Pericardial  cavity,  140 

Peripheral  nervous  system,  186 

Peristalsis,  157 

Peristome,  40 

Peritoneum,  59,  137 

Permian  period,  358 

Perspiration,  176 

Petal,  75,  107,  108,  112 

Petiole,  71 

Phagocyte,  338 

Pharynx,  123,  152,  153,  155 

Phenotype,  275,  277,  279,  281 

Phloem,  60,  76,  81,  87 

Phosphates,  36 

Photosynthesis,  35,  87,  332;  chem- 
ical equation,  36 

Phylogeny,  364 

Physcia,  332 

Physical  basis  of  life,  7-10,  400 

Physical  sciences,  1 

Physics,  origin  of,  395 

Physiological  division  of  labor 
(see  Division  of  labor) 

Physiologus,  383 

Physiology,  3,  4,  367;  compara- 
tive, 396;  Flowering  Plant,  84- 
90;  history,  394-398 

Pigeon,  compared  with  Archaeop- 
teryx,  360;  brain,  189;  dissec- 
tion of,  151;  domestic  varieties, 
373 

Pineal  body,  187,  188,  189 

Pinna,  152,  198 

Pistil,  75,  107;  compound,  108 

Pitcher-plant,  73 


Pith,  60,  76,  81,  87 

Pituitary  body,  183,  447 

Placenta,  205 

Planaria,  223 

Plant,  body,  61-90;  classification, 
413,  414;  chromosome  reduc- 
tion, 228;  colorless,  38;  evolu- 
tion, 105,  112-114;  food,  397; 
green,  30;  gross  structure,  65- 
75;  histology,  75-84;  ideal 
vertical  section,  76;  physiologi- 
cal activities,  87;  reproduction, 
91-114;  stem,  81;  unicellular, 
30,  39 

Plantigrade,  316 

Plasma,  163 

Plastid,  26,  27 

Plexus,  nerve,  190,  191 

Pliny  the  Elder,  382 

Pliocene  period,  358 

Polar  body,  231,  233 

Polar  lobe,  255 

Pole  cells,  126 

Pollen,  110,  112;  basket,  329; 
brush,  326;  combs,  329;  grain, 
108;  tube,  109,  110 

Pollination,  111 

Polygon,  frequency,  299-305 

Polyhybrid,  276 

Polymorphism,  218 

Polytrichum,  101 

Pond  Scum,  92,  413 

Population  and  pure  lines,  300 

Porpoise,  317,  416 

Portal  vein,  150,  167,  168 

Porto  Santo  Rabbits,  369 

Potato,  66,  70 

Potential  energy,  6,  15,  35,  42 

Precaval  vein,  164 

Preformation,  253,  257,  259,  401, 
406 

Pressure,  313 

Prickly  Pear,  70 


INDEX 


471 


Priestley,  398 

Primary  cylinder,  76 

Primates,  358,  416 

Pronephric  duct,  179 

Pronephros,  179 

Pronuclei,  241 

Prophase,  225 

Prosencephalon,  187,  188 

Prostomium,  123 

Proteins,  12,  14,  36,  42,  87,  157 

Proterozoic  era,  358 

Prothallus,  103,  104 

Protista,  214,  216;  fertilization 
in,  243-248 

Protohippus,  361,  862 

Protonema,  101,  102 

Protophyta,  39,  214 

Protoplasm,  4,  7-10,  19;  alveolar 
structure,  10;  appearance,  8; 
chemical  composition,  11;  con- 
cept, 400;  and  environment, 
9,  17 

Protoplast,  31 

Protopodite,  132 

Protozoa,  39,  116,  214;  classifica- 
tion, 414;  discovery,  388;  fer- 
tilization in,  335 ;  malarial  para- 
site, 335;  maturation  in,  233 
(see  Paramecium) 

Pseudopodium,  9 

Psychology,  4 

Psychozoic  era,  358 

Pteridophyta,  103,  112,  414 

Pubis,  144,  14$,  354 

Pulmonary  artery  and  vein,  150- 
153,  165,  171,  172 

Pulvillus,  328 

Pupil,  201 

Pure  lines,  300,  302-306;  versus 
population,  300,  305 

Purkinje,  7 

Pyloric,  caecum,  148;  valve,  156 

Python,  355 


R 

Rabbits,  Porto  Santo,  369 

Radial  symmetry,  118 

Radius,  144 

Rana,  149  (see  Frog) 

Ray,  390 

Reaumur,  395 

Recapitulation  theory,  364,  403 

Receptor-effector  system,  183,  184 

Receptor,  external,  193;  internal, 
193  (see  Sense  organs) 

Recessive  character  (see  Charac- 
ter) 

Recombinations,  268 

Rectum,  155 

Redi,  210,  388 

Reducing  division,  231  (see  Oogen- 
esis  and  Spermatogenesis) 

Reduction,  228,  229 

Reflex  action,  193 

Reflex  arc,  184 

Reflexes,  342 

Regeneration,  217;  in  Crayfish, 
Earthworm,  Flat  worm,  Para- 
mecium, Salamander,  Snail, 
221;  and  grafting  in  Hydra,  221 

Rejuvenation,  244 

Renaissance  science,  346,  384-386 

Renal  artery,  180 

Renal  portal  system,  164,  165,  167 

Reproduction,  11,  17,  212-222; 
in  animals,  203-208;  asexual, 
113;  biparental,  54;  versus 
fertilization,  243,  251;  in  plants, 
91-114;  uniparental,  54 

Reproductive  organs,  98-100; 
Crayfish,  131;  Earthworm,  204; 
Hydra,  204;  system,  116;  evo- 
lution of,  179,  208,  365;  Verte- 
brate, 137,  148-152 

Reptile,  117,  136;  versus  Bird, 
359;  brain,  189;  dissection  of, 
150 


472 


INDEX 


Respiration,  37,  87,  89,  396;    in 

animals,  43,  161-174;   chemical 

equation,  37;  in  Invertebrates, 

169;  in  Vertebrates,  169 
Respiratory,   currents,   paths    of, 

170;  membranes,  175;   system, 

116 
Response,  18;    organic,  307  (see 

Adaptation) 
Retina,  200,  201 
Reversion,  269 
Rhinencephala,  188  (see  Olfactory 

lobes) 

Rhizoids,  104 
Rhizome,  70,  74,  103 
Rib,  137,  141 
Ricinus,  81 
Rockweed,  62,  63 
Rodentia,  349 
Rods  and  cones,  201 
Roman  science,  382 
Root,  65-69,  76,  78-80,  112;  cap, 

79;  hair,  80;  primary,  65;    tip, 

79,  87 

Rostrum,  131 
Rotifer,  135;   parthenogenesis  in, 

249 
Round    Worms,    parthenogenesis 

in,  249,  415 
Runners,  70;   of  Strawberry,  69 


Sacculus,  196,  197 

Salivary  glands,  155,  157,  159 

Sap  cavity,  77 

Saprophytic,  51 

Sarcode,  7 

Sargassum,  64 

de  Saussure,  393 

Scales,  72 

Scapula,  141,  144 

Sceloporus,  150 

Schizogony,  335 


Schleiden,  399 

Schuchert,  C.,  358 

Schultze,  7,  400 

Schwann,  242,  399,  400 

Scientific  method,  2 

Sciurus,  152,  349 

Sclerotic  coat,  201 

Sea  Lettuce,  63 

Sea  Urchin,  135,  259,  415;  devel- 
opment, 58;  egg,  256 

Seaweeds,  62,  63 

Sebaceous  gland,  139 

Secondary  root,  66 

Secretion,  158 

Seed,  66,  110,  111,  112;  coat,  111; 
plants,  61 

Segregation,  274,  280-283,  286, 
290,  306 

Selaginella,  107 

Selection,  299-306,  377;  artificial, 
373;  natural,  306,  324,  374,  376, 
378,  410,  411;  in  population, 
300;  in  pure  lines,  300 

Semicircular  canals,  190-198 

Seminal  fluid,  152,  207 

Seminal  vesicle,  123,  151 

Senile  degeneration,  244 

Sense,  auditory,  196;  cutaneous, 
195;  cells,  differentiation  of, 
194;  organs,  116,  193-202; 
pain,  195;  sight,  198;  smell, 
196;  taste,  195;  temperature, 
195 

Sensitive  Fern,  74 

Sensory  nerve,  191 

Sepal,  107,  112 

Septa,  121 

Setae,  124 

Sex,  34,  113;  chromosome,  292- 
295,  377;  determination,  291- 
293;  differentiation,  96-98; 
linked  characters,  268,  291,  293; 
origin,  94,  95 


INDEX 


473 


Sexual  characters,  secondary,  206 

Shark,  164,  416 

Sheep,  263 

Shoot,  65,  76,  81 

Simplex  group,  234 

Sinus  venosus,  165,  166 

Skeleton,  appendicular,  143;  axial, 
143;  Bat's  wing,  352;  Bird's 
wing,  352;  Cat,  145;  Fish,  141; 
Gorilla,  354;  Horse's  leg,  352; 
Man,  354;  Man's  arm,  352; 
Ox's  leg,  352;  Vertebrate,  140- 
146;  Vertebrate  limbs,  352; 
Whale's  flipper,  352 

Skin,  138,  175;   human,  139 

Skull,  141,  143;  bones,  ^/'evo- 
lution, in  Camel,  363;  evolution, 
in  Horse,  362 

Sloth,  317 

Smell,  sense  of,  196 

'Smilax,'  70 

Snake,  416;  hind  limbs,  355 

'Social  heredity,'  297 

Sociology,  4 

Sol,  8 

Somatic  cells,  229,  246  (see 
Germ  plasm  and  Germinal 
continuity) 

Somatoplasm,  265 

Somite  (see  Metamere) 

Sorus,  104 

Spallanzani,  395 

Special  creation,  210,  346 

Species,  262,  345;  classification, 
391,  392;  concept,  390;  muta- 
bility, 410 

Specific  form,  11 

Spencer,  15,  114,  115,  375 

Sperm,  101,  104,  106,  112,  236; 
discovery,  388;  human,  239; 
Snake,  25 

Spermatic  fluid,  152,  207 

Spermatid,  231 


Spermatocytes,   primary,  second- 
ary, 231 

Spermatogenesis,    230-232;     dia- 
gram of,  231 

Spermatogonia,  224 

Spermatophytes,  61,  112,  414 

Sphaerella,  30-38,  52,  169,  413 

Spider,  129,  415 

Spinal  cord,   137,   142,   152,  153; 
paths  of  nervous  impulses,  193 

Spinal  nerves,  188,  190,  193 

Spines,  72 

Spireme,  225 

Spirogyra,  61 

Spleen,  137,  148,  149,  150,  152 

Splint  bones,  355,  361,  362 

Spondylomorum,  55 

Sponges,  115,  117,  414 

Spontaneous  generation,  209,  210, 
388 

Sporangium,  65,  74,  112 

Spore,  31, 101-113;  formation,  92, 
93,  312,  332 

Sporogony,  335 

Sporophyll,  65,  74,  106,  112 

Sporophyte,  64,  101,  104,  105 

Sporulation,  213,  335 

Squeteague,  food  of,  330 

Squid,  330,  415 

Squirrel,  349;    classification,  350; 
dissection,  152 

Stamen,  107,  108,  112 

Staminate  flower,  355 

Stapes,  198 

Starch,  13,  36 

Starfish,  221,  415 

Statolith,  131 

Stele,  76 

Stem,  60,  65,  69-71,  81,  103,  112 

Sternum,  145,  151,  153 

Stigma,  108 

Stimulus,  18 

Stipule,  71 


474 


INDEX 


Stoma,  28,  88 

Stomach,  137,  148-158,  155 

Storage  root,  67 

Struggle  for  existence,  330,  375, 

411 

Style,  108 

Subclavian  artery,  171;  vein,  164 
Subgenus,  351 
Suboesophageal  ganglion,  122, 123, 

130,  131 
Suborder,  351 
Subspecies,  351 
Sugar,  13,  36,  158 
Sulfates,  36 
Sulfur  Bacteria,  309 
Sundew,  73 

Survival  of  the  fittest,  375,  406 
Swammerdam,  388 
Sweat  gland,  139,  176 
Swimming  foot,  131,  132 
Sylvius,  395 
Symbiosis,  331-334;      Alga     and 

Fungus,  332 
Symmetry,  bilateral,  124;   radial, 

118 
Sympathetic  nervous  system,  186; 

~192 

Synapse,  185 
Synapsis,  281,  234,  235,  287,  289, 

290,  296 

Synaptic  mates,  232 
Syngamy,  244  (see  Fertilization) 
Synkaryon,  241,  244,  250 
Systems,  of  organs,  116 


Tactile  corpuscle,  195 
Talpa,  317 
Tamias,  349 
Tap  root,  65 
Tapeworm,  135,  414 
Tapirs,  distribution  of,  368 
Tarsus,  144,  145 


Taste,  sense  of,  195 
Taxonomy,  4,  348-351,  390-392 
Teeth,  138;   evolution,  in  Camel, 

363;  evolution,  in  Horse,  362 
Telophase,  225 
Temperature,   body,    174;    limits 

for  life,   311,   312;    regulation, 

174,  312;  sense,  195 
Tendril,  70,  72 
Tennyson,  203 
Tentacles  of  Hydra,  119 
Testis,  119,  123,  131,  149-152,  203 
Tetanus,  311 
Thallophytes,  112,  413 
Thallus,  62,  63,  112,  113 
Theophrastus,  2,  381 
Thistle,  72 
Thomson,  J.  A.,  6,  154,  297,  307, 

364 

Thoracic  duct,  168 
Thorax,  130,  140,  153 
Thrush,  369 

Thymus  gland,  150,  152,  155,  159 
Thyroid  gland,  152,  155,  159,  182 
Tibia,  144,  145,  354 
Time,  geologic,  358;  cosmic,  358 
Tissue,  59;    connective,  59,   142; 

systems,  60,  116 
Tonsils,  155 
Totipotent,  258 
Toxin,  338 

Trachea,  149,  153,  155 
Tracheid,  86 
Transpiration,  87,  88 
Transverse  process,  137,  143 
Treviranus,  3,  410 
Trichocyst,  40 
Tricuspid  valve,  165,  172 
Trihybrid,  280,  281 
Trillium,  70 

Trypanosome,  337,  338,  414 
Tunicate,  146,  415 
Turgor,  84 


INDEX 


475 


Turnip,  66 

Tympanic  membrane,  197,  193 

Typhlosole,  124 

U 

Ulna,  144,  145,  352,  354 

Ulothrix,  61,  94,  95,  96,  413 

Ulva,  63,  413 

Umbilical  cord,  205 

Underwing  Moth,  320 

Ungulata,  349 

Unguligrade,  316 

Uniformitarian  doctrine,  376,  406 

Unit  character,  262,  280,  282,  286, 
306 

Urea,  43,  176 

Ureter,  148-153,  179,  180 

Urinary  bladder,  137,  148-153, 
179 

Urinary  and  reproductive  systems, 
interrelationship,  179,  207 

Urogenital  canal,  207;  pore,  148; 
system,  207;  system  of  Verte- 
brates, 179 

Uropod,  132 

Urostyle,  149 

Uterus,  human,  205 

Utriculus,  196 


Vaccination,  339 

Vacuole,   26,   77;    contractile,   9, 

40;  food,  9;  gastric,  40 
Valves,  163,  165,  172 
Variation,    237,    261;     adaptive, 

378;    fluctuating,  301;    herita- 

bility  of,  264-269;   universality 

of,  410 

Varieties,  351 

Vascular  bundle,  77,  78,  83 
Vascular  plants,  64,  103 
Vascular  system  (see  Circulation) 


Vaso-motor  nerves,  174 

Vein,  163;  of  leaf,  77  (see  Circula- 
tion) 

Veinlets,  163 

Vena  cava,  152 

Venous  system,  166  (see  Circula- 
tion) 

Ventral  aorta,  164,  171 

Ventral  root,  192 

Ventricle,  163 

Vermiform  appendix,  153, 155,  355 

Vertebra,  137,  141,  145,  153; 
human,  143 

Vertebral  column,  141 

Vertebrates,  117,  135-153;  body 
plan,  136-138;  brains  of,  189; 
characters,  146,  147;  circula- 
tion and  respiration,  161-174; 
classification,  350,  416;  coelom, 
140;  labyrinth,  diagram  of, 
197;  limb,  plan  of,  144>  longi- 
tudinal section,  137;  skeleton, 
140-146;  skin,  138;  transverse 
section,  137;  urogenital  system, 
179 

Vesalius,  384,  385,  394 

Vespertilio,  318 

Vestigial  organs,  317,  355,  361,  362 

Violet  seed,  111 

Vitamines,  14 

Vitreous  humor,  201 

Vocal  organ  of  Bird,  151 

Volant,  316 

Volvox,  55,  56,  217 

de  Vries,  269 

W 

Walking-stick,  322 

Wallace,  410 

Warm-blooded  animals,  176  (see 
Homothermal) 

Waste,  nitrogenous,  43;  and  re- 
pair, 10 


476 


INDEX 


Water  Lily,  108 

Weismann,  267,  404 

Whale,  116;    skeleton  of  flipper, 

352 

Wheat,  263 
Wood,  88 
Working  hypothesis,  5,  396 


X  chromosome,  292,  293,  377 
Xylem,  60,  76,  81,  87 


Yeast,  SIS,  310 
Yolk,  238 


Zoogeography,  368 

Zoology,  3,  4 

Zygote,  32,  95,  98;  chromosomes 
in,  289,  290,  293;  in  genetics, 
275-284;  organization  of,  251- 
260 


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